“The soldiers were asking about the security situation and also making jokes and laughing,” said the store owner, who refused to give his real name for fear of reprisals from local militias. “Some of them said, ‘Be sure that we’ll come back again in order to buy clothes from you before we leave on vacation.’ ”

After the soldiers left the store, he said, he climbed up a ladder to a storeroom to retrieve his lunch and then heard a large explosion. He scrambled back down, he said, to find the bodies of two of the soldiers he had just been chatting with lying in the doorway of the store. Four of the soldiers died at the scene, and a fifth died later of his wounds.

Image Mourners in Basra carried a coffin containing the body of Dr. Khalid Nasir al-Mayah, who was killed Monday by gunmen. Credit... Atef Hassan/Reuters

Muhammad, a hamburger vendor whose stand is about 175 feet from the site of the bombing, said the same group of eight or nine American soldiers had been coming to the street for the last three days, getting out of their Humvees and walking around the shopping area, called the Rawad intersection after a popular ice cream parlor there.

“Usually, we see the Americans come in Humvees and they don’t stop, they just keep driving,” said Muhammad, who was afraid to give his last name. On Monday, he said, a soldier carrying a notebook walked into a currency exchange called The Ship. The other soldiers gathered in a small group.

“When the explosion happened we panicked and started running, and the gunner on one of the Humvees started shooting,” he said. “Even the Iraqi soldiers and police started firing in the air, so we jumped into one of the narrow alleys and remained there hiding until the Iraqi soldiers ordered us to walk at least two blocks away from the spot.”

He said that before the bombing, he had been surprised that the soldiers had allowed pedestrians to come up and talk to them, instead of keeping them at a distance as they normally do.