WASHINGTON—The Justice Department on Wednesday is expected to seek to join civil lawsuits stemming from the Gulf of Mexico oil spill, the first major federal legal action in the disaster, according to people familiar with the matter.

By joining the private litigation, Justice Department lawyers are positioning themselves to play a major role in the coming litigation, including depositions of key witnesses. That could aid the government's continuing probe into the disaster aimed at building a civil and possibly a criminal case against the companies involved, these people said.

The Justice Department declined to comment, as did BP PLC, which owned the blown out well.

The government's move is the first salvo of what is likely to be a lengthy and complex legal fight as it and the companies involved try to assign blame for the April 20 explosion of the Deepwater Horizon rig. The disaster killed 11 people and resulted in an estimated 4.9 million barrels of oil being spilled into the Gulf of Mexico before BP plugged the leak on July 15.

Dozens of private-party lawsuits have been consolidated in so-called multidistrict litigation in federal court in New Orleans, representing claims against BP and its contractors for damages from the worst offshore oil spill in U.S. history.