Agents who spent much of the day in Mellow's office left after removing several boxes that included documents sealed in plastic evidence bags, the Wilkes-Barre Times-Leader reported on its website Friday.
Mellow could not be reached for comment.
"Sen. Mellow is cooperating fully with all requests, and he is willing to provide any information that is deemed necessary to complete their inquiry," his press secretary, Lisa Scullin, said.
Scullin referred reporters to Mellow's longtime attorney, Sal Cognetti, who was out of town and not available for comment.
The raid occurred on the same morning that former State Rep. Mike Veon was sentenced in Harrisburg in the so-called Bonusgate scandal. The state Attorney General's Office is prosecuting cases in that investigation and had nothing to do with the Mellow raids, office spokesman Nils Frederiksen said.
Mellow, Democratic leader of the Senate and its longest-serving current member, announced this year that he would not seek reelection. He was first elected to the chamber in 1970.
The Inquirer reported last year that the Senate spent $213,000 in state money from 2001 to 2006 to rent Mellow's district office from a business half-owned by his then-wife, Diane. After they divorced, her share of the business went to Mellow, and he held it from 2007 until 2008.
Former longtime Mellow staff member Gabriel J. Giordano and his wife, Celestine, owned the company that held title to the property. In 2000, they made Diane Mellow a half-owner of the company.
Diane Mellow told The Inquirer in July that she had not paid for her stake in the company, that it was largely a mystery to her, and that she had no role in its operation.
Asked in July about the history of the company and its ownership, Sen. Mellow told The Inquirer that his recollection of the deal was "fuzzy" and that he was uncertain of the building's ownership before 2001. Gabriel Giordano had worked in the district office since 1973.
Mellow sold the building in 2008 for $350,000 but still occupies rental space there.
FBI agents interviewed Mellow's ex-wife in August at the federal building in Scranton about the rental payments and other business dealings, people familiar with the interview said.
Janine Pavalone, a lawyer for Diane Mellow, said her client had no comment on Friday's raids.
The state Ethics Commission, which has barred legislators from renting space from themselves at state expense, has been investigating the deal since an ethics complaint was filed against Mellow.
A 1978 state ethics law deems it a conflict of interest for public officials to use their offices for the private financial benefit of themselves, members of their immediate families, or businesses with which they or their relatives are associated. Violation entails a criminal penalty.
Contact staff writer John Sullivan at 215-854-2473 or email@example.com.