A car bomber detonated his explosives at a checkpoint protecting offices of Iraq's former prime minister and a Sunni lawmaker on Tuesday, killing two guards in a neighborhood bordering the fortified Green Zone.

Both men were out of the country, as is common for Iraqi politicians, many of whom maintain homes abroad and are frequently targets for assassination.

Tuesday's bombing took place in western Baghdad, less than a quarter-mile from a series of buildings that included offices of Ayad Allawi, Iraq's first post-Saddam Hussein prime minister and a secular Shiite, and those of Saleh al-Mutlaq, the head of the Iraqi National Dialogue Front, a Sunni bloc.

Al-Mutlaq, speaking from neighboring Jordan, said when the suicide bomber reached the first checkpoint "he claimed that he was an employee and had access."

The offices are in a residential neighborhood, but many of the homes were converted to work spaces because it is convenient to the Green Zone, where the Iraqi government has its headquarters.

Hussam al-Azawi, a member of Allawi's bloc, said there were indications of an assassination plot ahead of the suicide attack.

"The threats and plots came from a foreign country," al-Azawi told The Associated Press, without naming the country. "We received intelligence about this and informed the government and the Americans to reinforce the guards at our headquarters."

The car bomber turned off the main road and accelerated toward the checkpoint where the morning shift guards were gathered, police said. Al-Mutlaq confirmed reports by police and hospital officials that two guards were killed.

"Everyone is vulnerable," he told Al-Arabiya television. "We have been targeted by three groups — the Americans, Iraqi forces and a suicide bomber. Everyone should wake up and do something to change this situation."

In January, six Iraqis were killed in a U.S.-led raid on other offices for al-Mutlaq. The U.S. military and Iraqi police said they suspected the offices were being used as an al-Qaida safe house.

"I turned this house into an office for the Front after all my offices were attacked before," al-Mutlaq said. He said the attack was yet another demonstration that "it is time to rebuild the police and army."

Three Iraqis are on trial in Germany for plotting Allawi's death in 2004 during a visit to Berlin, including one convicted of supporting the radical Islamic group Ansar al-Islam. And in April, a suicide bomber slipped past security and into the Iraqi parliament dining hall in the Green Zone, killing a Sunni lawmaker and wounding seven other legislators.

Violence has declined drastically in Baghdad since this summer, when the influx of U.S troops to the capital gained momentum. But a spate of attacks in recent days has underscored the fragility of the gains.

Drive-by gunmen on motorcycles fatally shot the head of Iraq's largest psychiatric hospital as he was returning home from work late Monday, police and a Health Ministry official said, speaking on condition of anonymity because they feared reprisal.

Dr. Ibrahim Mohammed Ajil was the head of Rashad hospital, Iraq's largest and well-known mental institution, which lies on the outskirts of the sprawling Sadr City district of Baghdad.

According to figures from the Iraqi Health Ministry released earlier this year, 618 medical employees, including 132 doctors, as well as medics and other health care workers, have been killed nationwide since 2003.

Earlier Monday, mortar shells crashed into an Iraqi prison at an Interior Ministry complex, landing in a cellblock and killing at least five inmates.

And in other attacks this week, a roadside bombing targeting a police patrol in eastern Baghdad killed one policeman and wounded five other people on Monday, police and hospital officials said. And on Sunday, a roadside bombing struck a convoy carrying a popular provincial police chief with a reputation for cracking down on militias.

Still, the U.S. military has pointed to strong security gains, especially in the capital. In a statement Tuesday, the military said indirect fire attacks — a term for mortars and rockets — had declined in November to 25, compared to 49 in October. According to the statement, there were seven mortar or rocket attacks in Baghdad during the first week of December — all but one in residential neighborhoods.