An explosion rocked Bagram airfield, the largest US base in Afghanistan, early Saturday morning, killing at least four people and injuring 14 others, according to a NATO-led Resolute Support Mission press release.

“An explosive device was detonated on Bagram Airfield resulting in multiple casualties. Four people have died in the attack and approximately 14 have been wounded,” Resolute Support said in a statement.

Later in the day, US Defense Secretary Ash Carter confirmed the four killed were two American troops and two American contractors.

Update on Bagram explosion: 4 killed, approx. 14 hurt. Info to follow pic.twitter.com/sV0AHzGbz9 — Resolute Support (@ResoluteSupport) November 12, 2016

The blast was caused by a man wearing a suicide west, NBC News reported, citing a senior US official. The attacker chose the time and location because he “was looking for an opportunity to do the most damage,” the official added.

The Taliban has claimed responsibility for the attack, according to a statement of the group’s spokesman Zabihullah Mujahid on Twitter, Reuters reported.

Bagram Airfield is the largest US military base in Afghanistan and is located in the Parwan Province of Afghanistan. Bagram is currently maintained by the Combined Joint Task Force 1st Cavalry Division (CJTF-1). The base is mainly occupied by the International Security Assistance Force (ISAF), and occasionally used by the United States Armed Forces.

My thoughts go to the loved ones of those killed at #Bagram Airfield & the wounded. #NATO stands w/ #Afghanistan in the fight against terror — Jens Stoltenberg (@jensstoltenberg) November 12, 2016

The airfield has a dual runway capable of handling heavy military aircraft, such as the Lockheed Martin C-5 Galaxy or Russian Antonov An-225.

The American military presence in Afghanistan has been largely scaled down since the US invasion to oust the Taliban government in October 2001. The NATO combat mission in Afghanistan officially ended in 2014, when responsibility for the nation’s security was handed over to domestic forces. Approximately 10,000 US troops remain in the country as part of NATO’s “Resolute Support” mission.