Carefully tracking a yearslong U.S. attorney's probe into corruption at the United Auto Workers, General Motors Co. general counsel Craig Glidden and his outside counsel at Kirkland & Ellis began to see a pattern emerging.

On Wednesday the GM lawyers revealed what they saw in a federal racketeering lawsuit filed against Fiat Chrysler Automobiles NV. The suit accuses Fiat Chrysler of "a systemic and near decade-long conspiracy to bribe senior union officials to corrupt the collective bargaining process and labor relations," to GM's detriment.

In short, the suit alleges Fiat Chrysler paid bribes to corrupt the union bargaining process to win concessions and other favorable treatment not granted to GM, using the union to try and financially force GM into a merger.

The suit paints Fiat Chrysler's late CEO, Sergio Marchionne, as the mastermind behind the alleged conspiracy. For several years Marchionne had sought a merger with GM, but the company always rebuffed him.

Fiat Chrysler released a statement in response to the GM suit, saying, "We are astonished by this filing, both its content and its timing. We can only assume this was intended to disrupt our proposed merger with [French carmaker PSA Peugeot] as well as our ongoing negotiations with the UAW."

The statement added, "We intend to vigorously defend against this meritless lawsuit and pursue all legal remedies in response to it."

The company did not immediately respond to messages, but on Thursday Fiat Chrysler chairman John Elkann told reporters in Turin, Italy, that the Peugeot merger will move forward and that "we are not worried" by the lawsuit, according to the Associated Press.

At Detroit-based GM, Glidden was unavailable for comment. But in a statement after filing the suit, he said, "We have alleged in the complaint that Mr. Marchionne is responsible for conceiving, executing and sponsoring the alleged wrongdoing. We have no present intent to pursue the UAW. Our focus is on FCA. We believe the responsibility firmly rests on the orchestrater of the wrongdoing … and we plan to hold FCA responsible."

Glidden added, "This lawsuit is intended to hold FCA accountable for the harm its actions have caused our company and to ensure a level playing field going forward." The harm included unfair labor costs and other operational advantages, according to the suit.

The suit was filed in U.S. District Court in the Eastern District of Michigan, where the U.S. Attorney's Office has been conducting a multiyear investigation into union corruption. So far the investigation has led to convictions of seven union personnel and three Fiat Chrysler executives.

It was filed by Hariklia Karis, a litigation partner in Kirkland's Chicago office, and other Kirkland attorneys, along with the Detroit law firm Honigman. Karis did not immediately return messages Thursday.

The defendants are FCA US LLC, based in Auburn Hills, Michigan, and its parent company, Fiat Chrysler. GM also named the three convicted Fiat Chrysler executives—Alphons Iacobelli, Jerome Durden and Michael Brown—as co-defendants in the suit.

Hours after the suit was filed, UAW president Gary Jones resigned, though he is not mentioned in the complaint.

Earlier this month, federal prosecutors charged retired UAW vice president and former GM board member Joseph Ashton with fraud and money laundering as part of the investigation. He also is not mentioned in the GM complaint.

GM did not ask for a specific dollar amount, but the suit seeks damages, "including but not limited to the billions of dollars in damages GM suffered as a result of defendants' bribery scheme, pattern of racketeering, and other unlawful acts" plus punitive damages, restitution, costs and other fees.

In addition, the U.S. statute covering racketeering allows a court to treble the damage award against a losing defendant, raising a potential for tens of billions of dollars in damages.