Federal agents are investigating whether former Senate Majority Leader Trent Lott knowingly played a role in an alleged conspiracy in 2006 to influence a Mississippi judge presiding over a multimillion-dollar lawsuit against famed plaintiff attorney Richard "Dickie" Scruggs, according to people familiar with the situation.

Mr. Scruggs and several associates are scheduled to stand trial March 31 on charges that they offered $40,000 in bribes to State Court Judge Henry L. Lackey in return for a favorable ruling in a lawsuit against Mr. Scruggs over $26.5 million in legal fees.

Mr. Lott, who is a brother-in-law to Mr. Scruggs, unexpectedly announced his resignation from the Senate two days before Mr. Scruggs was indicted last November. Since then, Mr. Lott has been interviewed by federal agents at least once, according to a person familiar with the case.

The U.S. attorney's office in Oxford, Miss., which is leading the investigation, is also examining whether several associates of Mr. Scruggs induced a different Mississippi jurist, Hinds County Judge Robert Delaughter, to rule in favor of Mr. Scruggs in a separate lawsuit by promising that Mr. Lott would recommend Judge Delaughter for a seat on the federal bench.

The two lawsuits were about whether Mr. Scruggs owed former legal associates tens of millions of dollars in disputes over legal fees.