ALBANY, N.Y. — Assembly Speaker Sheldon Silver was arrested Thursday morning in Manhattan, but it’s not clear what charges he will face.

An FBI spokesman said Silver was taken into custody shortly before 8 a.m. at 26 Federal Plaza in Manhattan.

Silver, 70, has been been one of Albany’s most powerful figures since he was became speaker in 1994.

The spokesman said Silver’s arrest was the result of a long-term FBI investigation, but said he had no further information about Silver’s charges.

The New York Times, citing unnnamed sources, said federal prosecutors plan to charge Silver over significant income he received from a Manhattan law firm that was never listed on his state-mandated financial disclosure statements.

“I hope I’ll be vindicated,” Silver said as he entered the federal building, according to the Times. Silver’s arrest threatens to throw the state Capitol into an upheaval.

Silver’s arrest would cause an upheaval at the state Capitol, where the Manhattan Democrat has served since 1977. He wouldn’t have to resign as speaker over the charges, but he would face increased pressure to do so. His second in command is Majority Leader Joseph Morelle, D-Irondequoit, Monroe County.

A spokesman for U.S. Attorney Preet Bharara declined comment early Thursday. There was no immediate comment from Silver’s spokesman.

The New York Times reported Dec. 29 that federal authorities are investigating income Silver may have received from another law firm, Goldberg & Iryami, which specializes in real estate tax reductions for properties in New York City. The income hasn’t been listed on Silver’s annual financial disclosure forms.

Last year, he reported on disclosure forms earning between $650,000 and $750,000 in 2013 as of counsel at a powerful Manhattan law firm, Weitz & Luxenberg. But he didn’t list income from any other specific firm.

On Jan. 7, Silver was re-elected as speaker, and next year he would become the longest serving speaker in state history.

He declined Jan. 7. to discuss the case, but said he had not heard from prosecutors and that his attorneys “have not been directed to do anything.”

Asked by reporters for more clarification about his outside income, he declined comment.

“I don’t think it’s appropriate at this time under these circumstances to be commenting on these matters,” Silver said.

Assembly Democrats have previously brushed aside Silver’s troubles, saying he’s been a strong leader for the state and their conference.

Silver has faced numerous questions about his leadership, including sexual-harassment cases involving lawmakers under his watch. In 2012, Silver quietly agreed to a $103,000 settlement with two accusers of Assemblyman Vito Lopez, D-Brooklyn, and Silver has since said that the confidential settlement was wrong.

Assembly Republicans have called on him to resign as speaker over the sexual-harassment cases. But Assembly Democrats have continued to support him, saying he’s been a strong leader for the state and their conference.

Bharara has been investigating unfinished cases involving the now-defunct Moreland Commission that Gov. Andrew Cuomo abruptly shuttered last year. The commission had been looking at lawmakers’ outside incomes and campaign-finance reports to see if there were any illegalities or conflicts of interest.

Silver has long been criticized for his income from Weitz & Luxenberg, with accusations that he’s held up tort reform because the firm benefits from large personal injury settlements. He has dismissed the complaints.

Lawmakers need to be convicted of a felony to be forced to resign from office. Silver would follow a long line of lawmakers who have faced legal troubles in Albany.

Silver’s arrest would put the legislative session into turmoil. He is one of the so-called “three men in a room” who with the Senate president negotiate with the governor over the state budget due March 31 and legislation. The legislative session runs through late June.

Cuomo introduced his budget Wednesday to the Legislature, and Silver was present.

Spector reports for the Gannett Albany (N.Y.) bureau