The Defense Department warned its leaders of an imminent “mass casualty event” the day before a suicide bomber struck and killed 13 U.S. soldiers in Afghanistan, Politico reported.

In a virtual daily briefing on Wednesday morning, Defense Secretary Lloyd Austin instructed more than a dozen of the department’s top officials around the world to prepare for such an event, according to classified detailed notes of the meeting shared with Politico.

Thirteen U.S. soldiers were among 92 people killed during a suicide bombing at the Kabul airport on Thursday night.

Politico reported that, during the Pentagon briefing, chairman of the Joint Chiefs of Staff Gen. Mark Milley warned of “significant” intelligence indicating that the Islamic State’s Afghanistan affiliate, ISIS-K, was planning a “complex attack.”

Commanders from Kabul on the video conference relayed Milley’s message to U.S. military members at the Abbey Gate, where American citizens had been told to gather in order to gain entrance to the airport.

“I don’t believe people get the incredible amount of risk on the ground,” Austin said during the briefing, Politico said.

Although commanders, during a later call, detailed a plan to close Abbey Gate by Thursday afternoon Kabul time, Americans decided to keep open the gate longer to allow British allies to continue evacuating their personnel.

U.S. troops were processing entrants to the airport at Abbey Gate when the suicide bomb attack happened.

Politico based its report on classified notes from three separate calls, and interviews with two defense officials with direct knowledge of the calls.

The transcript of the conference calls, authenticated by a defense official, clearly showed that top officials were warning of a potential attack, narrowed down to a handful of potential targets, and a 24-48 hour time frame.

“U.S. forces at [the airport] were aware of and accounting for a variety of threats, and exercising extreme vigilance,” a Politico source said. “We took numerous actions to protect our forces and the evacuees, but no amount of effort will completely eliminate the threat of a determined enemy.”

Politico said it withheld information that could affect military operations at the Kabul airport. The Pentagon, however, slammed the news outlet for its report.

“This story is based on the unlawful disclosure of classified information and internal deliberations of a sensitive nature,” Pentagon spokesperson John Kirby said in a statement. “As soon as we became aware of the material divulged to the reporter, we engaged Politico at the highest levels to prevent the publication of information that would put our troops and our operations at the airport at greater risk.

“We condemn the unlawful disclosure of classified information and oppose the publication of a story based on it while a dangerous operation is ongoing.”

President Joe Biden and top administration officials previously had warned about the general threat ISIS posed to the airport. The president this weekend said an additional ISIS attack was “highly likely.”