Federal prosecutors in Brooklyn have been investigating whether several Ukrainian officials helped orchestrate a wide-ranging plan to meddle in the 2020 presidential campaign, including using Rudy Giuliani to spread their misleading claims about President Joe Biden and tilt the election in Donald Trump’s favor, according to people with knowledge of the matter.

The criminal investigation, which began during the final months of the Trump administration and has not been previously reported, underscores the federal government’s increasingly aggressive approach toward rooting out foreign interference in American electoral politics. Much of that effort is focused on Russian intelligence, which has suspected ties to at least one of the Ukrainians now under investigation.

The investigation is unfolding separately from a long-running federal inquiry in Manhattan that is aimed at Giuliani. While the two investigations have a similar cast of characters and overlap in some ways, Giuliani is not a subject of the Brooklyn investigation, the people said.

Instead, the Brooklyn prosecutors, along with the FBI, are focused on current and former Ukrainian officials suspected of trying to influence the election by spreading unsubstantiated claims of corruption about Biden through a number of channels, including Giuliani, Trump’s personal lawyer at the time. It is unclear whether the Brooklyn prosecutors will ultimately charge any of the Ukrainians.

At one point in the investigation, the authorities examined a trip Giuliani took to Europe in December 2019, when he met with several Ukrainians, according to the people, who spoke on the condition of anonymity to discuss an ongoing inquiry.

At least one of the current and former officials Giuliani met, a Ukrainian member of parliament named Andriy Derkach, is now a focus of the Brooklyn investigation, the people said.

The trip was the culmination of a yearlong effort by Giuliani, with support from Trump, to undermine Biden’s presidential campaign. The effort proceeded primarily on two parallel tracks: collecting information from Ukraine to attack Biden’s diplomatic efforts there as vice president, and pressing Ukraine to announce investigations into Biden and other Trump critics.

The effort ultimately backfired, leading to Trump’s first impeachment.

Amid the impeachment proceedings, U.S. intelligence officials warned Trump that Derkach was seeking to use Giuliani to spread disinformation. Giuliani, who has said he did not receive a similar warning at the time, continued to vouch for the authenticity of information he received, even after Trump’s Treasury Department imposed sanctions against Derkach for election interference, and accused him of being “an active Russian agent.”

In an interview last year, Giuliani said there was nothing to dissuade him from meeting with Derkach, who was not under sanctions at the time. “I have no reason to believe he is a Russian agent,” Giuliani said.

Today, Giuliani’s lawyer defended the search for information about Biden, disputing that he relied on misleading information. “When you investigate allegations of corruption, you talk to all sorts of people; some are credible, and some are not,” the lawyer, Robert J. Costello, said. He added that “some day the truth will come out” about Biden’s dealings in Ukraine.

Together, the Manhattan and Brooklyn investigations present a challenge for the Biden Justice Department, which has pledged to remain above the political fray even as it inherited a number of sensitive investigations linked to Ukraine and Russia.

The president’s son, Hunter Biden, for example, is facing a federal criminal tax investigation that appears to be partly related to work he did in Ukraine, and a Justice Department special counsel is investigating the origins of Robert Mueller’s investigation into Russia and Trump’s 2016 campaign.

The investigation into Giuliani in Manhattan — which entered an aggressive new phase last month when FBI agents executed search warrants at his home and office — centers on whether he lobbied the Trump administration to remove the U.S. ambassador to Kyiv on behalf of Ukrainian officials who wanted her gone. It is a violation of federal law to lobby the U.S. government on behalf of foreign officials without registering with the Justice Department, and Giuliani never registered.

Giuliani, who has not been accused of wrongdoing, has denied that he worked for any Ukrainians. He has cast his interactions with them as part of his effort to help Trump, and he denounced the FBI searches as a “corrupt double standard” by the Justice Department, which he said had ignored “blatant crimes” by Biden and other Democrats.

It is unclear why the Manhattan prosecutors are not also handling the investigation into the Ukrainians, though Trump’s Justice Department regularly clashed with that office and steered some cases involving Ukraine to the Brooklyn office instead.

A spokesman for the U.S. attorney’s office in Brooklyn declined to comment, as did spokesmen for the U.S. attorney’s office in Manhattan and the FBI.

The Brooklyn investigation is not a typical criminal case.

Inquiries into foreign influence operations are generally conducted by FBI counterintelligence agents, who are assigned to track foreign spies inside the United States and thwart their intelligence activities. But in this case, the agents and prosecutors are seeking to determine whether federal laws have been broken.

If the Brooklyn prosecutors ultimately decide to file charges, they could rely on the same lobbying law used in the Giuliani investigation, known as the Foreign Agents Registration Act, which may have required some of the Ukrainians to disclose efforts in the United States to influence the government or media. It is also a federal crime to conspire to defraud the United States, and prosecutors have used that law to charge Russians accused of meddling in previous elections.

It might prove difficult to arrest and extradite Ukrainians who face charges to the United States. Still, charges would most likely prevent them from traveling to most parts of the world, where they could be held for possible extradition.

The Treasury Department has already leveled economic sanctions for election interference against some of the Ukrainians, essentially preventing them from doing business in the United States, or through American financial institutions.

An initial round of sanctions in September took aim at Derkach, who the Treasury Department said has been “an active Russian agent for over a decade.” He was educated at the Dzerzhinsky Higher School of the KGB in Moscow, according to a Ukrainian biography.

Then, in January, the department levied sanctions against seven Ukrainians who it asserted were part of Derkach’s inner circle, including Andrii Telizhenko, a former Ukrainian government official who worked closely with Giuliani and Senate Republicans to provide damaging information about Biden and other Trump critics.

A representative for Derkach did not respond to requests for comment, but in the past, he has called the Treasury Department’s sanctions, issued under Trump, revenge by Biden’s “deep state associates.”

Between late 2019 and mid-2020, Derkach directed his efforts at a number of Americans, not just Giuliani. He “leveraged U.S. media, U.S.-based social media platforms, and influential U.S. persons to spread misleading and unsubstantiated allegations,” the Treasury Department said.

Giuliani began collecting information from Ukrainian officials in late 2018 to help Trump undermine Mueller’s investigation and impugn Biden over his son’s work for an energy company owned by a Ukrainian oligarch widely viewed as corrupt.

After the arrest on unrelated campaign finance charges of two Soviet-born businessmen who had helped Giuliani connect with Ukrainian officials, Giuliani increasingly turned for assistance to Telizhenko and Andrii Artemenko, a former member of the Ukrainian parliament who now lives in the Washington area and works as a lobbyist, according to people with direct knowledge of Giuliani’s efforts.

Federal investigators have interviewed Artemenko as part of the Brooklyn investigation into the Ukrainian officials, several people with knowledge of the matter said. He was not sanctioned and does not appear to be a target of the investigation, they added.

A lawyer for Artemenko, Anthony Capozzolo, declined to comment. Artemenko told Politico last month that he has spoken to the FBI about Giuliani, without elaborating further.

Telizhenko and Artemenko joined Giuliani in Washington in late November 2019 at the Washington studios of the conservative cable network One America News to record a series that the network claimed exposed corruption by the Biden family, and attacked “the impeachment hoax” as well as Mueller’s investigation.

Ahead of the December 2019 trip, Artemenko signed an agreement with Giuliani to help locate witnesses who would defend Trump, according to three people with knowledge of the contract, which Giuliani also signed. Artemenko was not paid for the work. Artemenko’s company later signed a lobbying contract to briefly represent Derkach in Washington.

In December 2019, Giuliani and a crew from the network traveled to Budapest and Kyiv to film interviews with current and former Ukrainian officials who claimed to have incriminating information about the Bidens and other Democrats.

Those who were interviewed included Derkach and two other men — another member of parliament and a recently fired Ukrainian prosecutor — who also ended up facing sanctions in January by the Treasury Department. The department described the two men, along with Telizhenko, as members of Derkach’s inner circle and accused them of advancing “disinformation narratives” to influence the 2020 presidential election.

It is unclear if Telizhenko is a focus of the Brooklyn criminal inquiry.

In an interview Thursday, Telizhenko rejected the accusations from the Treasury Department, saying that he had never met Derkach before Artemenko arranged the meeting over his objections. “I said, ‘This is not a good meeting,’” Telizhenko said he told Giuliani, adding that Derkach “was already toxic in Ukraine,” because “everybody knew that he was pro-Russian.”