BAGHDAD, Iraq – A suicide bomber detonated explosives yesterday inside a packed bus bound for a southern Shiite city, killing 32 people and wounding 44, police said. The blast pushed the three-day death toll from suicide attacks in the capital to at least 75.

Meanwhile, a statement posted on the Internet in the name of the Islamic Army in Iraq claimed it had killed an American hostage. The statement did not name him or provide photos, but the group earlier identified its captive as Ronald Alan Schulz and threatened to kill him unless all prisoners in Iraq were released.

The suicide attack occurred as the bus was pulling away from east Baghdad’s Nadhaa station bound for Nasiriyah, 200 miles to the south. A man carrying a bag suddenly jumped on the vehicle through the open door, apparently waiting until the last moment to board to avoid security checks.

He was challenged by the conductor but insisted on taking a seat, police Lieutenant Wisam Hakim said.

“He sat in the middle of the bus, and then the explosion took place,” Mr. Hakim said.

Police Lieutenant Ali Mitaab said 32 people were killed and 44 wounded. Most of those killed were on the bus, which was gutted by flames, but several people around a food stall also died, police said.

Officials at the scene said the death toll was especially high because the blast triggered secondary explosions in gas cylinders at the stall.

Several other explosions rumbled through the heart of the capital Thursday morning, including one that struck an American convoy killing an American soldier, the military said. The American command also said that a Marine was killed the day before in a bombing in Ramadi, 70 miles west of Baghdad.

The station, the main departure point for buses heading to the Shiite south, was the scene in August of a horrific triple car bombing that killed at least 43 people and wounded 89.

The White House press secretary, Scott McClellan, said he could not confirm the death of the American hostage. Mr. Schulz’s family in North Dakota said he was an electrician and was last heard from in Amman, Jordan. His brother, Ed, said he was advised by the State Department that Mr. Schulz might still be alive, and his sister, Julie, said the family was “just trying to get some information.”