Sen. Pat Toomey (R-Pa.). (AP)

Updated at 8:30 p.m.

Sen. Patrick Toomey's involvement in months-long negotiations over how to expand the nation's gun background check program began much sooner than this week, according to an accounting provided by staffers Thursday.

Toomey (R-Pa.) only emerged as a key player in recent days, but he first instructed his staff in March to reach out and determine how far along Sen. Joe Manchin III (D-W. Va.) was on a proposal to expand gun background checks to virtually all gun purchases. The process ended late Tuesday as Toomey and Manchin staffers hammered out the particulars of a deal over sandwiches from Taylor Gourmet — a staple for Philadelphia transplants working in Washington.

The two senators from neighboring states first met during a meeting in Manchin's office after Toomey joined the Senate in 2011. The two have worked together on issues of regional concern, including emissions standards and permitting for coal, aides said. They flew together in February to an energy conference in Pittsburgh, where they discussed ways they might be able to work together more often.