Attorney General Tom Corbett’s yearlong probe of legislative operations entered new turf this week when his investigators subpoenaed records from the state Senate’s majority Republican caucus.

The subpoena, confirmed Tuesday by the Senate Republicans’ general counsel, Stephen MacNett, is believed to be the first served in the Senate during the investigation.

Corbett’s office is investigating whether any of $3.6 million in once-secret bonus payments to legislative staffers by all four caucuses in 2005-06 were rewards for campaign work. Agents are also believed to be looking for evidence of campaigning on state time or using state resources.