An insurgent umbrella group that includes Al Qaeda in Iraq claimed Friday one of its "knights" carried out the parliament suicide bombing in Baghdad's Green Zone, as Iraqi lawmakers gathered in a defiant session on the Muslim day of prayer.

The Islamic State of Iraq said in an Internet posting that it had delayed claiming responsibility to allow its men time to flee. The attack killed a moderate Sunni lawmaker.

Iraqi officials, who spoke on condition of anonymity because the investigation was incomplete, said the bomber was believed to have been a bodyguard for a Sunni lawmaker. The officials did not name the lawmaker, who was not among the casualties.

In two separate attacks Thursday, three American soldiers and two Iraqi translators were killed south of Baghdad, the military said Friday. Eight soldiers were wounded.

On Friday, the U.S. military said "after further research and consultation with government of Iraq officials" it had determined that only one civilian had been killed in the attack and 22 were wounded in the attack on the parliament building. The U.S. on Thursday said as many as eight people were killed in the bombing.

"A knight from the state of Islam ... reached the heart of the Green Zone ... the temporary headquarters of the mice of the infidel parliament and blew himself up among a gathering of the infidel masters," the Islamic State said in the statement posted on one Islamic Web site commonly used by insurgents.

The SITE Institute, which tracks militant postings, said the claim appeared authentic. Some media reported on Thursday that Al Qaeda in Iraq had issued a claim an hour after the attack, but the Islamic Web forum on which it was reportedly posted was not among those used by the Islamic State in Iraq, so SITE cast doubt on its authenticity.

Parliament officials said the victim was Mohammed Awad, a moderate Sunni lawmaker. Seven of the wounded were members of parliament, the officials said.

Iraqi lawmakers gathered Friday in a rare and defiant session of parliament on the Muslim day of prayer. A red and white bouquet sat in Awad's place in the parliament chamber. Empty seats outnumbered people, though, as lawmakers took the podium one after another to denounce the bombing. One MP had his arm in sling and a woman wore a neck brace.

"The more they (terrorists) act, the more solid we become. When they take from us one martyr, we will offer more martyrs," Vice President Adil Abdul-Mahdi said. "The more they target our unity, the stronger our unity becomes."

But the turnout was low because of a weekly Friday ban on driving.

"Also the MPs' turnout is very low today because most of them are visiting those who were wounded by the blast," said Mohammed Abu Bakr, head of the parliament's media office.

The meeting began late and adjourned after about 90 minutes.

The parliament chamber bore no signs of damage, but cleanup had yet to begin elsewhere in the building, where investigators were still trying to determine who was behind the attack and how they penetrated the tightest security in Baghdad -- the heavily fortified Green Zone compound, which houses the U.S. Embassy as well as offices of the Iraqi government.

"The cafeteria is still not clean. There is still flesh of the bomber on the floor," Abu Bakr said.

Iraqi parliament speaker Mahmoud al-Mashhadani said Friday's session was "a clear message to all the terrorists and all those who dare try to stop this (political) process, that we will sacrifice in order for it to continue."

"We feel today that we are stronger that yesterday," he said. "The parliament, government and the people are all the same -- they are all in the same ship which, if it sinks, will make everyone sink."

Deputy Prime Minister Barham Saleh told lawmakers that the government "had received indications that this building would be targeted." Before the attack Thursday, security guards took the unusual precaution of using dogs to search inside the parliament building.

Saleh and other Iraqi officials met Thursday with the commander of all U.S. forces in Iraq, Gen. David Petraeus, and decided to put the Interior Ministry in charge of security at parliament, an Iraqi government spokesman said, adding that it was previously guarded by a private security company.

Lt. Gen. Ray Odierno, the No. 2 U.S. commander in Iraq, said the U.S. military will not take over security of the parliament building and would leave it up to the Iraqis.

"It doesn't help them for us to provide that security, they have to do that," Odierno said, adding that the investigation into the bombing was continuing.

"Frankly, yesterday was a bad day, a very bad day. But we're going to come back from that," he said, speaking from Camp Liberty in Baghdad in a conference call to Pentagon reporters.

State-run Iraqiya television's transmission was draped Friday in a black mourning banner. Regular programming aired, but the screen had a black stripe across the upper left hand corner.

Several TV channels replayed images of the moment of the attack and the minutes following: a flash and an orange ball of fire causing Jalaluddin al-Saghir, a startled parliament member who was being interviewed, to duck. Smoke and dust billowed through the area, and confused and frightened lawmakers and others could be heard screaming for help. Al-Saghir escaped injury.

A woman was shown kneeling over what appeared to be a wounded or dead man near a table and chairs. The camera then focused on a bloody, severed leg -- apparently that of the suicide bomber.

The breach of security at parliament -- along with another bombing the same day that destroyed a bridge across the Tigris River and killed at least 11 people -- struck a blow to a two-month-old U.S.-Iraqi effort to pacify the capital. Violence was down slightly in Baghdad, but the crackdown was insufficient to halt Thursday's spectacular attacks.

Nassar al-Rubaie, head of the parliamentary bloc allied with radical Shiite cleric Muqtada al-Sadr, accused the U.S. of lax security that allowed the bomber in.

"The occupation forces are in charge of security of this area. But no one dares to hold them responsible for this issue," he said.

"The problem of the occupation is not inside or outside this hall, it is for all Iraqi people. Why don't we hold them completely responsible?"