Sheldon Silver, speaker of the New York State Assembly, might be arrested by federal authorities on Thursday on corruption charges related to undisclosed payments from a Manhattan law firm, The New York Times reported, citing people familiar with the matter.

Although details of the specific charges against Silver, a Democrat who has held the speaker's office for over two decades, have not yet been revealed, the FBI is believed to have been investigating allegations that he failed to disclose “substantial” payments from Goldberg & Iryami -- a firm that reportedly specializes in challenging tax assessments and seeking tax reductions for property owners -- according to the Times report. The amount of payments received by Silver, reportedly made over several years, has not yet been disclosed.

The federal investigation against Silver reportedly stemmed from an inquiry initiated in April last year by prosecutors from the office of the U.S. Attorney Preet Bharara, who has been looking into lawmakers’ sources of outside income. The investigation reportedly began after the anti-corruption Moreland Commission was shut down by New York Governor Andrew Cuomo in March 2014.

Previously, Silver had listed an income of $650,000 from outside work, including consulting work for the personal-injury law firm Weitz & Luxenberg, in addition to his $121,000 salary as the assembly speaker. However, he has not disclosed the exact nature of his work for the New York-based law firm, according to a report by The Wall Street Journal.