State Department officials should enlist help from U.S. health agencies as they assess a series of mysterious attacks on American diplomats in Cuba, according to lawmakers frustrated with the pace of the investigation.

“We need to use all available resources to discover the medical cause and impact of what happened to our embassy personnel in Cuba,” House Foreign Affairs Chairman Ed Royce, R-Calif., and Democratic counterpart Eliot Engel of New York wrote in a Tuesday letter.

Engel and Royce directed that appeal not to the State Department, but to the federal government’s top doctors. They want the Center for Disease Control and Prevention, along with the National Institutes of Health, to "take a leading role” in a review of attacks that left 24 U.S. diplomats suffering hearing loss and other “cognitive issues” over the last year.

“Your expertise is needed now more than ever in determining what precisely happened to U.S. personnel in Cuba,” they wrote to NIH Director Francis Collins and CDC chief Brenda Fitzgerald.

The missive is the latest sign of congressional discontent with how Secretary of State Rex Tillerson’s team has responded to the attacks, which began last November but weren’t revealed publicly until August. The incidents have baffled the State Department, which withdrew most American staff from Havana and expelled most Cuban diplomats from the United States but hasn’t been able to explain who carried out the attacks or how.

“We don't have the real health experts of the United States government looking into it,” a congressional aide told the Washington Examiner while discussing the letter. “You have the doctors at the State Department looking at this, but that’s not really their role ... their role is to treat individuals at the State Department.”

House and Senate lawmakers in both parties have complained about the State Department’s response to the attacks. A bipartisan group of representatives asked the Government Accountability Office, the watchdog arm of Congress, to open a review of the State Department’s decision-making on the issue in the last several months. Engel added to the chorus last month with another letter asking for an accounting from the State Department.

That letter remains unanswered, a silence from Foggy Bottom that may have contributed to their decision to turn to NIH and CDC.

“As the committee awaits an update from the State Department on the investigation into sonic attacks on personnel working at the U.S. Embassy in Havana, Cuba, we write to urge [CDC and NIH] to offer to take a leading role in investigating the medical effects of these incidents and their potential cause,” Engel and Royce wrote.

The State Department did not reply to a request for comment.