A frantic, somber day in the West Wing as attacks in Kabul resulted in the death of at least 13 U.S. service members.

An already perilous withdrawal of U.S. personnel and allies from Afghanistan turned into something much darker on Thursday as the kind of catastrophe President Joe Biden had been warning about took place outside Kabul’s main airport.

A series of terrorist attacks left at least 13 U.S. service people dead and 15 wounded — the deadliest U.S. casualty event in Afghanistan since 2011.

It was the most devastating moment in Biden’s young presidency.

In its wake, U.S. officials remained steadfast that they would conclude the evacuation mission from the 20-year war, raising additional questions about Biden’s handling of the end of America’s longest war.

For those in the White House, Thursday was one of the most emotionally trying and frenetic days since taking office. As the first reports came in about explosions around Kabul, officials were confronted with a deluge of information, prompting senior officials to remind staffers to ferret out facts from the speculation and chatter. During one staff meeting, sniffles could be heard as various staffers fought back tears when they learned of the U.S. deaths, according to a person close to the situation. One White House official described the pace of the day’s events as overwhelming.

Biden himself hunkered down for hours with his national security team in the Situation Room and the Oval Office. He was in the latter, getting briefed around 2 p.m., as the phrase “Where is Joe Biden?” began trending on Twitter.

Along with Secretary of State Antony Blinken, Defense Secretary Lloyd Austin and Joint Chiefs of Staff Chair Mark Milley, the president received continuous updates on the situation throughout the day. Vice President Kamala Harris, who is traveling through Asia, was streamed into the Situation Room for the president’s morning meeting with his national security team. Later, one of her top aides announced that she’d be scuttling plans to make a campaign-related stop in California and instead return directly to D.C.

The White House was also in constant contact with Afghan commanders on the ground, according to an official, as it gamed out how the deadly attacks would impact the president’s Aug. 31 deadline for withdrawal.

On Thursday evening, Biden delivered an address in the East Room that was at times somber and tearful and, at others, calm and reflective. He honored the U.S. service people killed in action and conveyed two main goals: finishing the mission to evacuate all the Americans who want to leave and as many allies as possible within the time constraints and retaliating against those responsible for the attacks.

“We will not forgive. We will not forget,” Biden proclaimed. “We will hunt you down and make you pay.”