Kabul, Afghanistan ? Nasir Ahmad lay in a hospital bed, his right eye swollen and his cheek covered in a bloody bandage, hours after being pulled from the charred wreckage of a bus blasted apart by a suicide bomber.

An Afghan police investigation team inspects the police bus after a bomb blast Sunday in Kabul, Afghanistan. A bomb ripped through a police bus in a crowded civilian area in the Afghan capital Sunday, killing 35 people, 22 of whom were police instructors, and wounding more than 50 others, officials said.

Ahmad, a 22-year-old janitor at Kabul’s police academy, gave short, pained responses as he described sitting in the back of the bus, behind at least 22 police trainers who couldn’t escape.

“There were between 30 to 40 police instructors in the bus,” Ahmad said, halting wearily after every other word. It was the only full sentence he managed to utter before stopping from exhaustion.

The deadliest insurgent attack since the U.S.-led invasion of Afghanistan in 2001 destroyed a bus full of police instructors at Kabul’s busiest transportation hub, killing 35 people and wounding 52, officials said.

Sunday’s enormous suicide blast, which raised the specter of an increase in Iraq-style bombings with heavy casualties, was at least the fourth attack against a bus carrying Afghan police or army soldiers in Kabul this year. The blast sheared off the bus’ metal sidings and roof, leaving a charred frame.

“Never in my life have I heard such a sound,” said Ali Jawad, a 48-year-old who was selling phone cards near the scene of the bombing. “A big fireball followed. I saw blood and a decapitated man thrown out of the bus.”

Condemning the attack, President Hamid Karzai said the “enemies of Afghanistan” were trying to stop the development of Afghan security forces, a key component in the U.S.-NATO strategy of handing over security responsibilities to the Afghan government one day, allowing Western forces to leave.

Purported Taliban spokesman Qari Yousef Ahmadi said a Taliban suicide bomber named Mullah Asim Abdul Rahman caused the blast. His claim could not be verified.

Zemeri Bashary, the spokesman for the Interior Ministry, said late Sunday that 35 were killed and 52 wounded in the blast. Karzai’s office said 22 police instructors died, indicating that 13 of the dead were civilians.