NEW YORK >> Federal investigators in Manhattan executed a search warrant today at the Upper East Side apartment of Rudy Giuliani, the former New York City mayor who became President Donald Trump’s personal lawyer, stepping up a criminal investigation into Giuliani’s dealings in Ukraine, three people with knowledge of the matter said.

One of the people said the investigators had seized Giuliani’s electronic devices.

Executing a search warrant is an extraordinary move for prosecutors to take against a lawyer, let alone a lawyer for a former president, and it marks a major turning point in the long-running investigation into Giuliani.

The federal authorities have been largely focused on whether Giuliani illegally lobbied the Trump administration in 2019 on behalf of Ukrainian officials and oligarchs, who at the same time were helping Giuliani search for dirt on Trump’s political rivals, including President Joe Biden, who was then a leading candidate for the Democratic presidential nomination.

The U.S. Attorney’s office in Manhattan and the FBI had for months sought to secure a search warrant for Giuliani’s phones.

Under Trump, senior political appointees in the Justice Department repeatedly sought to block such a warrant, The New York Times reported, slowing the investigation as it was gaining momentum last year. After Merrick Garland was confirmed as Biden’s attorney general, the Justice Department lifted its objection to the search.

While the warrant is not an explicit accusation of wrongdoing against Giuliani, it shows that the investigation has entered an aggressive new phase. To obtain a search warrant, investigators need to persuade a judge they have sufficient reason to believe that a crime was committed and that the search would turn up evidence of the crime.

Spokesmen for the FBI and the U.S. attorney’s office declined to comment.

The investigation of Giuliani grew out of a case against two Soviet-born men who aided his mission in Ukraine to unearth damaging information about Biden and his son Hunter, who was on the board of a Ukrainian energy company. The prosecutors charged the two men, Lev Parnas and Igor Fruman, with unrelated crimes in late 2019 and a trial is scheduled for October.

While investigating Giuliani, prosecutors have examined, among other things, his potential business dealings in Ukraine and his role in pushing the Trump administration to oust the American ambassador to Ukraine, which was the subject of testimony at Trump’s first impeachment trial.

As he was pressuring Ukrainian officials to investigate the Bidens, Giuliani became fixated on removing the ambassador, Marie Yovanovitch, whom he saw as an obstacle to those efforts. At the urging of Giuliani and other Republicans, Trump ultimately ousted Yovanovitch.

As part of the investigation into Giuliani, the prosecutors have explored whether he was working not only for Trump, but also for Ukrainian officials or businesses who wanted the ambassador to be dismissed for their own reasons, according to people briefed on the matter.

Under the Foreign Agents Registration Act, or FARA, it is a federal crime to try to influence or lobby the U.S. government at the request or direction of a foreign official without disclosing it to the Justice Department.

The prosecutors have scrutinized Giuliani’s dealings with Yuriy Lutsenko, one of the officials who helped Giuliani and his associates in their dirt-digging mission while also urging them to work to get the ambassador removed.

Among other things, the prosecutors have examined discussions Giuliani had about taking on hundreds of thousands of dollars in apparently unrelated consulting business from Lutsenko, which resulted in a draft retainer agreement that was never executed.

Giuliani has said he turned down the deal, which would have involved him helping the Ukrainian government recover money it believed had been stolen and stashed overseas.

As the investigation heated up last summer, prosecutors and FBI agents in Manhattan were preparing to seek the search warrant for Giuliani’s records about his efforts to remove the ambassador, but they first had to notify Justice Department officials in Washington, according to people with knowledge of the matter.

Federal prosecutors must consult Justice Department officials in Washington about search warrants involving lawyers because of concerns that they might obtain confidential communications with clients. The proposed warrant for Giuliani was particularly sensitive because his most prominent client was Trump.

Career Justice Department officials in Washington largely supported the search warrant, but senior officials raised concerns that the warrant would be issued too close to the election, the people with knowledge of the matter said.

Under long-standing practice, the Justice Department generally tries to avoid taking aggressive investigative actions within 60 days of an election if those actions could affect the outcome of the vote.

The prosecutors in Manhattan tried again after the election, but political appointees in Trump’s Justice Department sought once more to block the warrant, the people with knowledge of the matter said. At the time, Trump was still contesting the election results in several states, a legal effort being led by Giuliani, those officials noted.