The group is seeking details about Lynch's communication with a Clinton campaign aide, Amanda Renteria, as well as copies of documents and information about whether the FBI investigated the alleged communication.
The letters, sent Thursday, are signed by Senate Judiciary Chairman Charles Grassley, Ranking Democrat Dianne Feinstein, as well as Sens. Lindsey Graham and Sheldon White House, the chair and ranker on the subcommittee on Crime and Terrorism.
The senators question Open Society Foundations' Leonard Benardo and its General Counsel Gail Scovell, as well as Renteria and Lynch, about a May 24 story from the Washington Post that reported Lynch assured Renteria that she would not let the FBI investigation into Clinton go too far. Lynch declined to comment at the time.
An email reportedly recounting that alleged conversation and authored by Rep. Debbie Wasserman Schultz, who served at the time as DNC chair, was allegedly hacked by Russia, though the FBI later discounted its reliability.
The inquiry comes as the Senate Judiciary Committee examines the firing of former FBI Director James Comey, who President Trump says he dismissed due in part to his handling of the Clinton email probe.