A suicide bomber slipped through the tightest security net in Baghdad and blew himself up in the midst of lawmakers having lunch in the parliament dining hall Thursday. US officials said eight people, including parliament members, were killed in the deadliest-ever attack in the American-guarded Green Zone.

The stunning breach of security, on the same day that a massive bombing destroyed one of Baghdad's main bridges, laid a cloud of heavy doubt about progress in the latest US-Iraqi bid to clamp off violence in the capital. The drive has put thousands of troops on the streets in a massive operation to round up militants and their weapons.

A news video camera captured the moment of the blast, about 2:30 p.m. - 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 and to find colleagues. Al-Saghir reportedly escaped injury.

But 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. At least two lawmakers were among the dead.

Three miles north and seven hours earlier, a bombing sent a major bridge linking east and west Baghdad plunging into the Tigris River. Several cars plummeted into the murky, brown water, and at least 10 people were known to have died. Many more were believed missing.

Police blamed a suicide truck bomber for the attack on the al-Sarafiya bridge, which the British built in the 1950s. AP Television News video, however, showed the bridge broken in two places - perhaps the result of two blasts.

Security officials at Iraq's parliament said they believed the bomber in the cafeteria attack was a bodyguard of a Sunni lawmaker who was not among the casualties. The bombing, which wounded both Sunnis and Shiites, showed that determined suicide assailants remain capable of striking at will.

US military spokesman Maj. Gen. William Caldwell said the attack bore the trademarks of al-Qaida in Iraq. The terrorist group is fighting not only to oust US forces from Iraq but also against fellow Sunnis in the west of the country who have begun to leave the insurgency and side with US and Iraqi soldiers.

"We don't know at this point who it was. We do know in the past that suicide vests have been used predominantly by al-Qaida," the US military spokesman said.