WASHINGTON >> The FBI has significantly increased its use of stings in terrorism cases, employing agents and informants to pose as jihadis, bomb-makers, gun dealers or online “friends” in hundreds of investigations into Americans suspected of supporting the Islamic State, records and interviews show.

Undercover operations, once seen as a last resort, are now used in about two of every three prosecutions involving people suspected of supporting the Islamic State, a sharp rise in the span of just two years, according to a New York Times analysis. Charges have been brought against nearly 90 Americans believed to be linked to the group.

The increase in the number of these secret operations, which put operatives in the middle of purported plots, has come with little public or congressional scrutiny, and the stings rely on FBI guidelines that predate the rise of the Islamic State.

While FBI officials say they are careful to avoid illegally entrapping suspects, their undercover operatives are far from bystanders. In recent investigations from Florida to California, agents have helped people suspected of being extremists acquire weapons, scope out bombing targets and find the best routes to Syria to join the Islamic State, records show.

Officials said in interviews that because social media has given extremists a cloak of anonymity, these undercover stings — both online and in person — have become increasingly vital to gathering evidence and deterring possible attacks inside the United States.

“We’re not going to wait for the person to mobilize on his own timeline,” said Michael B. Steinbach, who leads the FBI’s national security branch. He added that the FBI could not afford to “just sit and wait knowing the individual is actively plotting.”

Counterterrorism officials said the Islamic State had inspired loyalists to strike quickly, even within days or weeks of their radicalization. Unlike wiretaps or searches, undercover operations do not require a judge to sign a warrant. They are overseen by FBI supervisors and Justice Department prosecutors, and so can usually be started more quickly.

But defense lawyers, Muslim leaders and civil liberties advocates say that FBI operatives coax suspects into saying and doing things they might not otherwise do — the essence of entrapment.

“They’re manufacturing terrorism cases,” said Michael German, a former undercover agent with the FBI who teaches law at New York University. In many of the recent prosecutions, he said, “these people are five steps away from being a danger to the United States.”

Karen J. Greenberg, the director of the Center on National Security at Fordham University, said undercover operations had become the norm for the FBI in the most recent Islamic State cases, with little debate or understanding of how the bureau actually conducts its investigations, especially its online stings.

“I think the FBI is really going down the wrong path with a lot of these ISIS cases,” she said, using a common acronym for the Islamic State, also known as ISIL.

Court records and interviews give a glimpse of the aggressive undercover methods that have become typical in investigations once the FBI identifies someone as a possible threat.

In Rochester, N.Y., a paid informant went undercover and drove a man suspected of being an Islamic extremist, Emanuel Lutchman, to a Wal-Mart in December to buy a machete, ski masks, zip ties and other supplies for a would-be terrorist attack on New Year’s Eve. Because Lutchman, a mentally ill panhandler, had no money, the informant covered the $40 cost.

In North Carolina, an undercover agent pressed another suspect, Justin Sullivan, on whether he was willing to commit acts of terrorism for the Islamic State — “do you think you can kill?” the agent asked in one online message — before giving him a silencer for an AR-15 assault rifle in June 2015.

And in Washington state, an undercover informant paid $1,100 to Daniel Franey, a former soldier, for acting as a lookout on several trips to buy duffel bags filled with assault weapons for a possible attack last summer.

The FBI arrested all three suspects before any attacks occurred, and has used similar undercover techniques to prosecute dozens of others it believes had ties to the Islamic State, court records show. While a number of defendants have pleaded guilty, most are still awaiting trial.

At least 40 agencies use covert operatives to investigate everything from tax cheating and welfare fraud to Supreme Court demonstrations. But in no agency have undercover stings been more central than in the FBI

While the FBI’s internal guidelines, last updated in 2011, require the “least intrusive” methods possible in investigations, bureau officials said they believed less intrusive methods might be impossible because suspects have the ability to remain anonymous on encrypted online sites.

“When the bad guys turn to encrypted areas, we’re dark, and the only way to gain a better understanding of what we’re up against may be through an undercover,” Steinbach said. He said using undercover agents online allowed the FBI to “flesh out” suspects by gaining their trust and persuading them to disclose their real identities.

The FBI has about 1,000 open investigations into “homegrown violent extremists,” which it defines as Americans motivated by a foreign terrorist group, including the Islamic State, to conduct attacks at home, officials said. They said a “significant number” of cases — hundreds in all — had entailed undercover operations against people suspected of being Islamic extremists, but that the FBI did not have precise numbers.

But court records examined by The Timesindicate that the FBI has used undercover operations with increasing frequency in its Islamic State investigations since the earliest cases emerged in March 2014.

Only about 30 percent of the first few dozen prosecutions through late 2014 appear to have relied on evidence gathered through undercover operations. That number climbed to about 45 percent by early last year, with a string of undercover prosecutions in New York, Minnesota and Illinois. And since February 2015, about 40 of 60 Islamic State prosecutions, or 67 percent, have been based on undercover operations.

The number of Islamic State prosecutions overall has slowed since January; officials believe a spate of prosecutions late last year may have deterred plotters. But undercover stings have remained the norm. So far this year, eight of the dozen Islamic State prosecutions have relied on undercover operatives, court records indicate.

In the most recent case, prosecutors two weeks ago charged a Bronx man, Sajmir Alimehmeti, 22, with traveling to Europe twice to try to fight with the Islamic State. He met with at least three undercover agents during the FBI’s investigation.

Undercover activity also appears to have risen in the hundreds of Islamic State investigations run by the FBI in the last two years, but it is not as sharp a spike as seen in those cases that led to formal charges, officials said.

Asked about The Times’ data, FBI officials said any increase in undercover activity probably reflected the emergence of what they believed to be a larger and more dangerous pool of Islamic State suspects. The officials said there had been no deliberate decision or policy change to rely more heavily on undercover agents.

Often, the targets in the stings first attracted the FBI’s attention for espousing what agents saw as support for the Islamic State or violent extremism online, on social media sites like Facebook or Twitter. The FBI insists that its agents comb these sites not as “thought police,” but rather to look for people who are threatening acts of terrorism. Once the FBI has identified someone as a potential threat, supervisors often have an undercover agent contact the suspect online.

In northern Illinois, for instance, an undercover agent sent a friend request on Facebook to Hasan Edmonds, a National Guardsman who had published posts on social media that the FBI had deemed alarming. Over the next several months, Edmonds and the undercover agent exchanged numerous online messages, discussing the possibility of traveling to the Middle East to help the Islamic State, or perhaps mounting an attack at the Illinois armory where Edmonds trained.

“Oaths have been given,” Edmonds wrote to an informant in one online message in January 2015, according to a criminal complaint. “When the time is right we will strike.”

A few days later, Edmonds wrote that he and a cousin were anxious to help the cause and “would love to do something like the brother in Paris did” — a reference to the terrorist attack a few weeks earlier on the satirical newspaper Charlie Hebdo.

The undercover agent responded by suggesting to Edmonds that if he could not make it overseas to fight with the Islamic State, he could show his commitment and achieve martyrdom in the United States, court filings said.

The online messages led to a meeting in person in March 2015 between a second undercover operative and Edmonds’ cousin, Jonas Edmonds, prosecutors said.

Weeks later, the cousins were arrested at Chicago Midway International Airport — with Hasan Edmonds apparently on their way to the Middle East — and charged with terrorism-related offenses. Both men have pleaded guilty and are facing long prison sentences.

In that case and in many others, FBI undercover operatives have let plots play out until a suspect acquires a weapon, for instance, or buys a plane ticket to Turkey en route to Syria.

An investigation that ended last month in Miami followed this pattern, with the normal tableau of undercover operatives and talk of a bloody attack. The FBI received word — exactly how is unclear — that James Gonzalo Medina, 40, a Muslim convert who also went by James Muhammad, might have been considering attacking a synagogue, according to court records.

It opened an investigation into Medina in March, but the initial evidence was spotty. At one meeting with an undercover informant and several other men, Medina accused a friend of wanting to blow up a synagogue; he seemed to distance himself from the idea.

Five days later, as Medina was driving with the FBI informant in a suburb of Miami, he pointed out a synagogue with what he called “David’s triangle star” on the outside. The FBI informant suggested attacking the synagogue during a Jewish holiday two weeks later, according to court files.

“Now that’ll be a good day to go and bomb them,” Medina responded, according to the files.

The informant took the proposed plot a step further, introducing Medina to an acquaintance the informant said was experienced with explosives. The acquaintance was actually an undercover FBI agent, who questioned Medina several times about why he wanted to bomb the synagogue.

Medina said he wanted to carry out a bombing and claim responsibility on behalf of “ISIS in America.”

“You need to be sure, brother,” the undercover agent told him, adding, “You know you don’t have to do any of this.” Medina reaffirmed his commitment, according to the court files.

Finally, in late April, the agent met Medina in a parking lot and gave him the bomb, which was inert and “posed no danger to the public,” prosecutors said. The two men drove toward the synagogue a few miles away, and after Medina stepped out of the car with the bomb in hand, he was arrested.

Officials said they were keenly aware that such cases would almost inevitably lead defense lawyers to charge that the FBI had illegally entrapped their clients. That is why, the officials said, they often gauge suspects’ commitment to carrying out an attack and give them an “out” to back away.

FBI officials are also quick to point out that, despite dozens of challenges to undercover terrorism cases brought since the Sept. 11, 2001, attacks, a judge has yet to throw one out on the grounds of entrapment.

Not that some judges have not considered it.

“I believe beyond a shadow of a doubt that there would have been no crime here, except the government instigated it, planned it and brought it to fruition,” Judge Colleen McMahon of the U.S. District Court in Manhattan said in 2011 in a case involving four Muslim men in Newburgh, New York.

The FBI planted an undercover informant inside a mosque in Newburgh as part of what became an elaborate, nearly yearlong plot to launch a Stinger surface-to-air missile at a local air base and two synagogues. The FBI even built a fake Stinger missile and had it delivered to the men.

McMahon said she was troubled by the FBI’s conduct, but she upheld the charges. So did the 2nd U.S. Circuit Court of Appeals, finding that the FBI’s conduct did not amount to entrapment.

The stings have left many Muslim leaders wary, even as the FBI has sought to build bridges to Muslim Americans. At mosques in Oregon, imams sometimes warn of FBI informants and caution “that we have those among us who are not with us,” said Tom Nelson, a Muslim lawyer in Portland who has represented a number of local men in terrorism-related cases.

His message for his Muslim friends, Nelson said, is blunt.

“Avoid the FBI like the plague,” he said. “They’re definitely not an ally. That’s what the FBI does — they infiltrate.”

© 2016 The New York Times Company