Jack Abramoff, one of K Street’s top Republican rainmakers, has stepped down from lobbying shop Greenberg Traurig amidst a Senate investigation into the multi-million dollar fees that he charged several American Indian tribes since 2001.

Richard Rosenbaum, a top officer of Greenberg Traurig, said in a statement released late Tuesday evening that Abramoff resigned after he “disclosed to the firm for the first time personal transactions and related conduct which are unacceptable to the firm.”

Greenberg Traurig also has retained Henry F. Schuelke III of Janis, Schuelke and Wechsler to “conduct a comprehensive investigation of these matters so the firm can take any additional action that may be appropriate.”

“The firm will finish its internal investigation before making further public comment,” Rosenbaum added.

Abramoff, in his own statement, said that recent press reports about “my lobbying practice has distracted from my efforts on behalf of my clients whose interests are now, and have always been, my number one priority.”

He added: “Therefore, I have determined that the best solution is to resign effective immediately.”

The lobbyist said he has not made a decision about his future.

“As for what I will do, I have several options on the table, but have not decided from which option to chose,” Abramoff said.

Abramoff’s departure comes after The Washington Post reported that Abramoff had raked in about $15 million in federally reported lobbying fees from four tribes in the last few years and that his business associate Mike Scanlon, a former spokesman for House Majority Leader Tom DeLay (R-Texas), reaped another $30 million from three of the same tribes.

At least one of the tribes is now complaining that they were bilked.

In response, Sen. John McCain (R-Ariz.), a member of the Senate Indian Affairs Committee, as well as Rep. Frank Wolf (R-Va.), have called for hearings on Capitol Hill.

Abramoff leaves Greenberg Traurig just three years after joining the Miami-based law firm.

In his brief stay, Abramoff helped vault Greenberg Traurig’s lobbying practice into one of the most lucrative on K Street.

Greenberg Traurig reported $25.5 million in lobbying fees last year, a six-fold increase over the firm’s lobbying revenues before Abramoff joined the firm in 2001.