Frustrated lawmakers are pressuring Defense Secretary Lloyd Austin for information about an attack last year in Kenya that killed three Americans, saying it is “simply unacceptable” for the Pentagon to stonewall Congress.

The lawmakers said in a letter sent to the Pentagon on Friday that the Defense Department has not responded to numerous requests for information since the Jan. 5, 2020, attack was sprung by militants on a seaside airfield near the Somali border, and that members of the House Committee on Oversight and Reform expect more from the Pentagon and U.S. Africa Command.

“On February 5, 2020, the Subcommittee on National Security wrote to DOD requesting a briefing on the attack,” the lawmakers wrote. “Seventeen months later — despite repeated requests to your office and AFRICOM — DOD has not provided any substantive information about the attack or the security lapses that contributed to it, citing ongoing investigations.”

The letter was signed by Rep. Stephen F. Lynch (D-Mass.), the subcommittee’s chairman, and Rep. Glenn Grothman (Wis.), its top Republican.

The attack was carried out by al-Shabab, an al-Qaeda affiliate. It began before dawn and spiraled into hours of violence.

Army Spec. Henry J. Mayfield Jr., 23, and defense contractors Bruce Triplett, 64, and Dustin Harrison, 47, were killed, and at least two other American defense contractors were wounded on a base in Manda Bay. Six contractor-operated aircraft also were destroyed or damaged, U.S. military officials said at the time. They were believed to include important surveillance aircraft used over Somalia.

The attack highlighted continued U.S. vulnerabilities in Africa, and marked the deadliest attack on Americans since a 2017 disaster in Niger, when four U.S. soldiers were killed after their unit was overrun by militants on a mission in which they had limited support.

In the aftermath of the Manda Bay attack, senior U.S. military officials launched an investigation to scrutinize what happened and how such a significant attack was allowed to happen. They did so as the Trump administration was openly considering ending the U.S. military’s counterterrorism mission in Somalia.

In April, Pentagon spokesman John Kirby announced that the investigation was complete. But rather than accept Africa Command’s assessment of its actions, Austin directed an independent review of the investigation.

Kirby said at the time that the Pentagon would not make any announcements about the investigation’s findings until the independent review — led by Paul Funk, the commanding general of Army Training and Doctrine Command — was complete.

About a week later, Army Gen. Stephen J. Townsend, who leads Africa Command, told the Senate Armed Services Committee that Austin had given the military “a target date of 90 days to get the report out.” Nothing has been released.

A Defense Department spokesman, Marine Lt. Col. Anton Semelroth, said in an email that the Pentagon is aware of the lawmakers’ request. The review that Austin directed “will provide added insight, perspective and the ability to assess the totality of this tragic event involving multiple Military Services and Department of Defense components,” Semelroth wrote.

Austin wants to ensure there is a full examination and consideration of the contributing factors in the attack, and that appropriate action is taken to reduce the possibility of a similar situation in the future, Semelroth added.

“The families impacted deserve nothing less,” he said.

Semelroth declined to say whether anyone has been fired as a result of the investigation, or if any of its findings already have been acted upon. Funk “will take the time he deems appropriate” to complete the independent review, and updates will be provided to the families of those who died and to Congress before the public is briefed, he added.

Lynch and Grothman said in their letter that it is Congress’s “solemn constitutional duty” to conduct oversight of U.S. military operations, and that they expect the Pentagon will “produce” the initial investigation report to their subcommittee by Friday, as well as the independent review once it is complete.

The lawmakers also requested a briefing about the investigation and subsequent review by Sept. 30.