BAGHDAD - Four bombs ripped through Shiite neighborhoods in Baghdad Thursday evening, killing at least 40 people in the worst violence the capital has seen in months, Iraqi officials said. An American civilian aid specialist working to improve education in Iraq was killed in a separate attack.

The violence underscored the fragile nature of the security gains in Iraq at a time when American forces are preparing to withdraw by the end of this year and the challenges facing the State Department personnel and American contractors who would continue on after the US military is gone.

The first three bombs went off in quick succession in a southwestern Baghdad neighborhood shortly after 7 pm. One targeted a Shiite mosque, another exploded just outside a popular market, while the third went off inside the market where people were doing their evening shopping ahead of the Muslim weekend, Iraqi police officials said.

The officials said 34 people died and 82 others were injured in the three blasts. An official from Baghdad's Yarmouk hospital confirmed the casualty figures.

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An Iraqi resident, Jabir Ali, said he was about 200 yards (meters) away when one of the bombs went off near a barber shop where his cousin works.

"I saw many people killed and injured. I went to see my cousin. The glass at his shop was broken and he was injured in his head, chest and hand by the glass," said Ali, who drove his cousin to the hospital.

About an hour later, a parked car bomb targeting a police patrol killed six people, including one policeman and five bystanders in a different neighborhood in southwestern Baghdad, said hospital officials.

The officials all spoke on condition of anonymity because they were not authorized to speak to reporters.

There was no immediate claim of responsibility, but Sunni extremists such as al-Qaida in Iraq generally tend to target Shiite mosques and neighborhoods and Iraqi security forces.

It was the worst attack in the capital since a parked car bomb exploded near a mourning tent in a northern Baghdad neighborhood in January, killing 48 people.

The American civilian killed earlier Thursday was Dr. Stephen Everhart, said a US State Department spokeswoman, Victoria Nuland.

"Dr. Everhart was an American citizen who was working in Iraq for an implementing partner of the United States Agency for International Development's Mission in Iraq. He was killed while working on a project to introduce a new business curriculum to a Baghdad university in a program supported by the Iraqi Ministry of Higher Education," she said in a statement.

"We are saddened by this tragedy and extend our thoughts and prayers to Dr. Everhart's family and loved ones, and to the three other injured victims and their families," she said.