Afghanistan bombing

A suicide bomber rammed his car into a U.S. convoy in Kabul on May 16,

killing at least 15 people including two American soldiers and four

civilian contractors, officials said. The brazen attack made May the

deadliest month this year for coalition forces. (AP)

A U.S. soldier arrives to the scene where a suicide car bomber attacked a NATO convoy in Kabul, Afghanistan, Thursday, May 16, 2013. A Muslim militant group, Hizb-e-Islami, claimed responsibility for the early morning attack, killing many in the explosion and wounding tens, police and hospital officials said. The powerful explosion rattled buildings on the other side of Kabul and sent a pillar of white smoke into the sky in the city's east. (AP Photo/Anja Niedringhaus)

Afghan policemen stop people from approaching the scene where a suicide car bomber attacked a NATO convoy in Kabul, Afghanistan, Thursday, May 16, 2013. A Muslim militant group, Hizb-e-Islami, claimed responsibility for the early morning attack, killing many in the explosion and wounding tens, police and hospital officials said. The powerful explosion rattled buildings on the other side of Kabul and sent a pillar of white smoke into the sky in the city's east. (AP Photo/Anja Niedringhaus)

An Afghan Army soldier peers through a destroyed shop window at the scene where a suicide car bomber attacked a NATO convoy in Kabul, Afghanistan, Thursday, May 16, 2013. A Muslim militant group, Hizb-e-Islami, claimed responsibility for the early morning attack, killing many in the explosion and wounding tens, police and hospital officials said. The powerful explosion rattled buildings on the other side of Kabul and sent a pillar of white smoke into the sky in the city's east. (AP Photo/Anja Niedringhaus)

An Afghan man directs his children away from the scene where a suicide car bomber attacked a NATO convoy in Kabul, Afghanistan, Thursday, May 16, 2013. A Muslim militant group, Hizb-e-Islami, claimed responsibility for the early morning attack, killing at least six people in the explosion and wounding more than 30, police and hospital officials said. The powerful explosion rattled buildings on the other side of Kabul and sent a pillar of white smoke into the sky in the city's east. (AP Photo/Anja Niedringhaus)

Afghan and U.S. soldiers arrive to the scene where a suicide car bomber attacked a NATO convoy in Kabul, Afghanistan, Thursday, May 16, 2013. A Muslim militant group, Hizb-e-Islami, claimed responsibility for the early morning attack, killing at least six people in the explosion and wounding more than 30, police and hospital officials said. The powerful explosion rattled buildings on the other side of Kabul and sent a pillar of white smoke into the sky in the city's east. (AP Photo/Anja Niedringhaus)

Afghans comfort a man, left, who claims he has lost all his children at the scene where a suicide car bomber attacked a NATO convoy in Kabul, Afghanistan, Thursday, May 16, 2013. A Muslim militant group, Hizb-e-Islami, claimed responsibility for the early morning attack, killing at least six people in the explosion and wounding more than 30, police and hospital officials said. The powerful explosion rattled buildings on the other side of Kabul and sent a pillar of white smoke into the sky in the city's east. (AP Photo/Anja Niedringhaus)

An Afghan fireman stands next to the debris of a car at the scene where a suicide car bomber attacked a NATO convoy in Kabul, Afghanistan, Thursday, May 16, 2013. A Muslim militant group, Hizb-e-Islami, claimed responsibility for the early morning attack, killing many in the explosion and wounding tens, police and hospital officials said. The powerful explosion rattled buildings on the other side of Kabul and sent a pillar of white smoke into the sky in the city's east. (AP Photo/Anja Niedringhaus)

Afghan men clean up the debris after a suicide car bomber attacked a NATO convoy in Kabul, Afghanistan, Thursday, May 16, 2013. A Muslim militant group, Hizb-e-Islami, claimed responsibility for the early morning attack, killing many in the explosion and wounding tens, police and hospital officials said. The powerful explosion rattled buildings on the other side of Kabul and sent a pillar of white smoke into the sky in the city's east. (AP Photo/Anja Niedringhaus)