A spokesman for the mayor did not respond to questions on Friday morning.
Prosecutors from the Manhattan U.S. attorney's public corruption unit are looking at whether people who helped raise money for de Blasio's 2013 election campaign and a non-profit organization that his advisers operated received favorable treatment from the mayor or his aides at City Hall, according to news reports.
The New York Times previously reported that the feds suspect de Blasio may have accepted donations from major NYC business players and developers during his 2013 run for mayor - some of them funneled through his "Campaign for One New York" nonprofit - in exchange for return favors while he's been in office.
The mayor's office and the US attorney's office both declined to comment.
Appeals court rules Second Amendment doesn't protect right to assault weapons
That fact, the court said, put them within the category of weapons the Heller opinion excepted from Second Amendment protection. Assault weapons are a class of semi-automatic firearms; large-capacity magazines can hold as many as 100 rounds of ammunition.
Exclusive CBS2 video shows de Blasio being driven from his attorneys' offices, his meeting there Thursday lasting roughly five hours.
The mayor is also under investigation by the Manhattan District Attorney's office in connection with state senate campaigns he assisted in 2014.
He used to be on the advisory board of the Mayor's Fund to Advance New York but is now is no longer serving on it. The newspaper said the mayor will meet with authorities without any immunity protections, but added that he had not been subpoenaed testify. The investigator was appointed by Democratic Gov. Andrew Cuomo, a frequent de Blasio rival.
De Blasio was met by a mob of reporters as he arrived to the 9 a.m. meeting in a black, unmarked police SUV, according to Newsday.