WASHINGTON (Reuters) — American forces launched a drone strike in Kabul on Sunday that killed a suicide car bomber suspected of preparing to attack the airport, U.S. officials said, as the United States nears the end of its military presence in the city.

The strike, first reported by Reuters, is the second carried out by U.S. forces in Afghanistan since an Islamic State suicide bomber struck the airport on Thursday, killing 13 U.S. troops and scores of Afghan civilians trying to flee the country.

One U.S. official said Sunday’s strike was carried out by an unmanned aircraft and that secondary explosions following the strike showed the vehicle had been carrying a “substantial amount of explosive material.”

Witnesses reported a large blast shaking a neighborhood north of Hamid Karzai International Airport, and television footage showed black smoke rising into the sky.

U.S. Central Command confirmed the strike and said it was investigating reports of civilian casualties.

“We know that there were substantial and powerful subsequent explosions resulting from the destruction of the vehicle, indicating a large amount of explosive material inside that may have caused additional casualties,” it said.

“It is unclear what may have happened, and we are investigating further,” Central Command added.

President Joe Biden warned on Saturday that the situation on the ground in Kabul remained extremely dangerous, and that his military chiefs had told him another militant attack was highly likely within the next 24-36 hours.

U.S. officials had said they were particularly concerned about the local affiliate of Islamic State (ISIS-K) attacking the airport as American troops depart, in particular the threat from rockets and vehicle-borne explosives.

Sunday’s drone strike took place as Biden headed to Dover Air Force Base in Delaware to honor the U.S. service members killed in Thursday’s suicide bombing.

On Friday the U.S. military launched a drone strike that it said targeted ISIS-K militants in Nangarhar Province, east of Kabul, killing two of the group’s planners and wounding a third.

With the United States continuing to withdraw troops, officials say concerns about another Islamic State attack will mount.

As the Taliban swept across Afghanistan earlier this month, Biden sent thousands of troops to Kabul airport to help evacuate American citizens, at-risk Afghans and other foreigners who wanted to escape the country’s new rulers.

At the peak of the deployment, there were 5,800 U.S. troops securing the airport, where the unprecedented airlift operation is set to end by Tuesday.

Despite Biden’s vow to go after the perpetrators of Thursday’s attack, U.S. officials have cautioned that the United States could do little to degrade ISIS-K beyond a symbolic act or limited operation.