A grand jury has been impaneled in the federal investigation into Rudy Giuliani

• Rudy Giuliani's assistant has been served a subpoena to appear before a federal grand jury, per ABC News.
• The news marks a significant turning point in the ongoing criminal investigation into Giuliani.
• Giuliani's apartment and office were also raided on Wednesday, and the feds seized his electronic devices.

A grand jury has been impaneled in the federal investigation into former New York mayor and Trump lawyer Rudy Giuliani, his lawyer told ABC News.

The revelation came after The New York Times reported that FBI agents had executed a search warrant on Giuliani's Manhattan apartment and office earlier Wednesday. Giuliani's electronic devices were seized as was a computer belonging to his longtime assistant, Jo Ann Zafonte. Bob Costello, Giuliani's lawyer, later told ABC News that the feds also served Zafonte with a subpoena to testify before the grand jury in May.

Executing a search warrant on a lawyer is an extraordinary step that requires approval from a federal magistrate judge, and Wednesday's developments mark an aggressive turning point in the longrunning criminal investigation into whether Giuliani violated foreign lobbying laws in his dealings with Ukraine.

The Times first reported in 2019 that the Manhattan US attorney's office was conducting a criminal probe into Giuliani's business dealings. Specifically, investigators are said to be examining whether he violated the Foreign Agents Registration Act (FARA), which requires that American citizens notify the Justice Department of any contacts they have with foreign governments or officials, and if they interact with the US government or media at the direction of those officials.

Prosecutors zeroed in on Giuliani as part of a broader investigation into two of his Ukrainian business associates, Lev Parnas and Igor Fruman. The two men were arrested in October 2019 on suspicion of trying to funnel foreign money into a pro-Trump super PAC and other entities to gain leverage in US political circles. Prosecutors also alleged that they tried to improperly influence US-Ukraine relations.

CNN reported that investigators from the Manhattan US attorney's office approached Kevin McCallion, a lawyer in New York, earlier in 2019 to ask about Giuliani's link to Parnas and Fruman. The investigation into the former New York mayor is also said to include a counterintelligence aspect, which indicates that the FBI may view him as a national security threat.

Giuliani, for his part, spearheaded a months-long, behind-the-scenes effort to influence US policy vis-a-vis Ukraine, an endeavor that ultimately led to then President Donald Trump's first impeachment in 2019.

He was also in contact with several Russian and Ukrainian political operatives as part of a public quest to dig up dirt on then candidate Joe Biden and his son, Hunter, ahead of the 2020 election. Giuliani's actions were so alarming to US officials that, according to The Washington Post, they warned the White House that Russian intelligence agencies were using Giuliani to funnel disinformation to Trump.

One of Giuliani's associates, Andriy Derkach, was recently sanctioned by the US and described as an "active Russian agent." Amid news of Derkach's sanctioning, Giuliani acknowledged in an interview with The Daily Beast that there was "50/50" chance he'd worked with a Russian spy and shrugged off the matter.

After news of the FBI's raids broke Wednesday, Giuliani's attorney described them as "legal thuggery," adding, "Why would you do this to anyone, let alone someone who was the associate attorney general, United States attorney, the mayor of New York City and the personal lawyer to the 45th president of the United States?"