State Sen. Carl Kruger is expected to surrender to federal authorities on Thursday morning to face corruption charges.

The probe threatens to topple a veteran Brooklyn Democrat, who as recently as last year commanded one of the Legislature's most influential posts. The expected charges would make him the third state senator to face corruption allegations in less than four months.

The investigation appears to stem from an earlier corruption probe of a former assemblyman, Anthony Seminerio, who pleaded guilty to accepting bribes from health-care executives in Queens, according to a lawmaker with knowledge of the probe. Mr. Seminerio died in January while serving out a federal prison sentence. Federal court documents from the case described a friendly relationship between Mr. Kruger and the chief executive of MediSys, a health-care network that includes Jamaica Hospital in Queens and Brookdale University hospital in Brooklyn.

The chief executive of MediSys sought the help of Mr. Kruger and Seminerio to help get approval in Albany for a hospital merger. At the time, MediSys and Parkway Hospital were vying to acquire two failing Queens hospitals, St. John's and Mary Immaculate, according to testimony from the case.

"I need your help with that...you and Kruger together are my two, uh, my two guys," MediSys chief executive David Rosen told Seminerio, according to transcripts from a 2008 wiretapped conversation.