The White House says it does not know how a suicide bomber was able to get close to a U.S. military base in Afghanistan where Vice President Dick Cheney was staying.
A spokesman, Tony Snow says the case is still being investigated.
Cheney was not hurt in the blast Tuesday outside the U.S. military base at Bagram. But the attack killed at least 22 people, including one U.S. and one South Korean soldier.
The Taleban has claimed responsibility for the blast and said it was targeting Cheney.
Cheney was staying at the base after his Monday meeting in Kabul with Afghan President Hamid Karzai was postponed due to bad weather.
After the explosion, the U.S. vice president said he was briefly moved to a bomb shelter. He later made a trip to Kabul to meet with Mr. Karzai, before leaving for Oman.
Military officials say 27 people, mostly Afghans, were wounded in the attack at the U.S. base.
Cheney arrived in Afghanistan Monday after a stop in Pakistan, where he expressed concern to President Pervez Musharraf that Taleban insurgents are regrouping in remote areas of Pakistan for a new offensive in Afghanistan.
The Taleban has declared that 2007 will be the bloodiest year for foreign troops since U.S.-led forces drove the extremist group from power in 2001.