(Reuters) – A U.S. drone strike killed a suicide car bomber who Pentagon officials said was preparing to strike Kabul airport on Sunday, as American forces worked to complete a withdrawal that will end two decades of military involvement in Afghanistan.

The strike was the second by the U.S. military since an Islamic State suicide bomb outside the airport on Thursday killed 13 U.S. troops and scores of Afghan civilians desperate to flee the country’s new Taliban rulers.

The airport has been the scene of a massive airlift by U.S. and allied forces evacuating their citizens and at-risk Afghans that is due to wind up ahead of a Tuesday deadline set by U.S. President Joe Biden.

Officials said the strike targeted suspected militants from ISIS-K, a local affiliate of Islamic State that is an enemy of both the West and the Taliban movement that seized power on Aug. 15 after a lightning offensive.

One U.S. official said it was carried out by an unmanned aircraft and that secondary explosions showed the target had been carrying a substantial amount of explosives. Television footage showed black smoke rising into the sky.

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

Biden said on Saturday that his military chiefs had told him another militant attack was highly likely.

The drone strike took place while remaining civilians waited at Hamid Karzai International Airport to be flown out before the last troops leave, a Western security official said. A U.S. official told Reuters on Saturday that fewer than 4,000 U.S. troops remained.

The Taliban said they had started their own investigations into the U.S. strike and whether the target was really a suicide bomber driving a vehicle loaded with explosives.

The United States and allies have taken about 114,400 people – including foreign nationals and vulnerable Afghans – out of the country in the past two weeks, but tens of thousands who want to go will be left behind.

“We tried every option because our lives are in danger. They (the Americans or foreign powers) must show us a way to be saved. We should leave Afghanistan or they should provide a safe place for us,” said one woman outside the airport.

The airlift – one of the biggest such evacuation operations ever – marked the end of a 20-year Western mission in Afghanistan that began when U.S.-led forces ousted a Taliban government that had provided safe haven for the perpetrators of the Sept. 11, 2001, attacks on the United States.

The final chapter came after the United States and the Taliban made a deal last year to withdraw foreign troops. The Western-backed government and Afghan army then melted away as Taliban fighters swept across the country earlier this month.

A Taliban official told Reuters the Islamist group had engineers and technicians ready to take charge of the airport.

One U.S. official said it was carried out by an unmanned aircraft and that secondary explosions showed the target had been carrying a substantial amount of explosives. Television footage showed black smoke rising into the sky.

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

Biden said on Saturday that his military chiefs had told him another militant attack was highly likely.

The drone strike took place while remaining civilians waited at Hamid Karzai International Airport to be flown out before the last troops leave, a Western security official said. A U.S. official told Reuters on Saturday that fewer than 4,000 U.S. troops remained.

The Taliban said they had started their own investigations into the U.S. strike and whether the target was really a suicide bomber driving a vehicle loaded with explosives.

The United States and allies have taken about 114,400 people – including foreign nationals and vulnerable Afghans – out of the country in the past two weeks, but tens of thousands who want to go will be left behind.

“We tried every option because our lives are in danger. They (the Americans or foreign powers) must show us a way to be saved. We should leave Afghanistan or they should provide a safe place for us,” said one woman outside the airport.

The airlift – one of the biggest such evacuation operations ever – marked the end of a 20-year Western mission in Afghanistan that began when U.S.-led forces ousted a Taliban government that had provided safe haven for the perpetrators of the Sept. 11, 2001, attacks on the United States.

The final chapter came after the United States and the Taliban made a deal last year to withdraw foreign troops. The Western-backed government and Afghan army then melted away as Taliban fighters swept across the country earlier this month.

A Taliban official told Reuters the Islamist group had engineers and technicians ready to take charge of the airport.