Jim Gaffigan and Olivia Thirlby have rounded out the cast of “Chappaquiddick” starring Jason Clarke as Sen. Ted Kennedy.

Kate Mara, Ed Helms, and Bruce Dern also star in the film directed by John Curran.

Sierra/Affinity has come on board to oversee international sales of the project. Apex Entertainment’s Mark Ciardi and Campbell McInnes are producing the film, which is financed by Apex. DMG’s Chris Cowles and Chris Fenton are executive producing, along with Taylor Allen and Andrew Logan, who wrote the screenplay featured on the 2015 Black List. Principal photography of the film commences Sept. 7 in Boston.

“Chappaquiddick” revolves around the tragic car accident that took the life of Mary Jo Kopechne (Kate Mara) when Senator Ted Kennedy (Clarke) crashed his car off a one-lane bridge in the summer of 1969. The young Kennedy escaped the car and the scene of the accident, leaving the 28-year-old passenger to drown. Kennedy went home, but didn’t report the accident, which took place on the eve of the Apollo 11 moon landing, for nine hours. The U.S. Senator struggled over the course of the next week between following his moral compass or using his power and influence to save his presidential aspirations and protect the Kennedy family legacy and name.

Gaffigan will play District Attorney Paul Markham and Thirlby will portray a member of the so called “boiler room girls” — a group of single women who attended the party preceding the accident on Chappaquiddick Island. They had all met while working together on Bobby Kennedy’s presidential campaign in 1968.

Gaffigan is represented by UTA, Brillstein and Rob Greenwald. Thirlby is represented by CAA, 3 Arts Entertainment, and Hansen Jacobson Teller Hoberman. WME serves as the film’s North American sales agent.