WASHINGTON—The Washington Metropolitan Area Transit Authority has filed a suit alleging breach of contract and failing in its fiduciary duty against Aon Corp. and several of its subsidiaries in connection with insurance coverage for a 2009 crash that killed nine people.

The suit, initially filed in District of Columbia Superior Court, was moved to federal district court this week. WMATA, which runs the Metro mass transit system in the Washington area, claims Aon did not inform it of potential coverage gaps.

“We believe that Aon failed to advise us to exercise an option that would have reduced our financial exposure after a claim was settled in 2008,” said a spokeswoman for WMATA in an e-mail. “We believe that we should have been able to avoid $9 million in expenses had Aon met its contractual obligations. The excess liability insurance program brokered by Aon included an option to reinstate aggregate limits, which it failed to advise us to exercise, leaving Metro to pay $9 million in claims arising out of the 2009 collision at Fort Totten.”

The June 2009 crash occurred when two Metro trains collided at the Fort Totten station northeast of downtown Washington during the evening rush hour. Nine people died, and more than 70 others suffered injuries in the deadliest crash in the system’s history.

The Metro spokeswoman said the suit was not about paying claims, because “we have been and will continue to meet our responsibility and obligations to the victims and families of that terrible tragedy.” But she added that “if Aon had not failed in its duties,” the claims would have come from insurance rather than Metro’s operating funds.

“We filed this suit like any responsible corporation would do—in a businesslike manner to hold an insurance brokerage accountable for expertise it was paid for, but failed to provide,” said the spokeswoman.

“We feel the claims are without merit and we look forward to defending ourselves in court,” said a spokesman for Aon in Chicago in an e-mail.

Several insurers, including American International Underwriters Inc., XL Capital Ltd. and underwriters at Lloyd’s of London, participated in Metro’s insurance program.