The U.S. military was re-examining security measures at bases across Iraq (search) on Thursday, a day after saying an attack that killed 22 people at a camp near Mosul (search ) was likely carried out by a homicide bomber who may have had inside information.

The explosion on Monday at the tightly guarded U.S. base raised questions about how the attacker infiltrated the compound, which is surrounded by blast walls and barbed wire and watched by U.S. troops who search every person going in and check his identity.

However Iraqis do a variety of jobs at the base, including translation, cleaning, cooking, construction and office duties.

A spokesman for the U.S. military command in Baghdad (search) said Thursday security measures are subjected to changes when needed.

"It is a fluid situation where our security measures and plans are constantly being adapted and reworked," said 1st Sgt. Steve Valley.

The apparent sophistication of Tuesday's operation — one of the deadliest single attacks on U.S. troops since the war began — indicated the attacker probably had inside knowledge of the base's layout and the soldiers' schedule. The blast came at lunchtime.

"We always have force protection keeping their eyes out," Lt. Col. Paul Hastings, spokesman for Task Force Olympia (search), the main force that controls northern Iraq, said Thursday. "For somebody that wants to take his life and kill himself, its very difficult to stop those people."

Asked how will they act following the attack, Hastings said that now that the cause of the attack is known, "a full investigation is now ongoing and from that full investigation we will act according."

At the Pentagon Wednesday, Gen. Richard Myers, the chairman of the Joint Chiefs of Staff, said a homicide bomber had apparently strapped an explosive device to his body and entered the dining hall where the blast occurred.

In western Baghdad, a U.S. soldier was killed and two were wounded by a roadside bomb, the U.S. command said. The attack occurred at 8:00 a.m. Thursday and the victims were members of the U.S. Army's Task Force Baghdad, which is in charge of security in the Iraqi capital.

The death raises the number of U.S. troops who have died since the start of the war in March 2003 to at least 1,322 members, according to an unofficial count by The Associated Press.

Early Thursday, hundreds of U.S. troops, Iraqi National Guards and Kurdish militiamen were seen in the streets of Mosul moving around in Bradley Fighting vehicles. In some eastern neighborhoods such as Muharebeen, Nour, Karama and Hadbaa they entered homes in search of weapons. One of the city's five bridges over the Tigris River reopened Thursday, after all were blocked off by U.S. troops on Wednesday.

Schools remained closed but more people were seen in the streets compared with the previous day. Iraqi National Guards manned a checkpoint near another U.S. base, the former palace of Saddam Hussein, stopping passing cars and searching them.

Residents said they were worried about the worsening situation in their city, which has seen a sharp upsurge of rebel activity in the past several months.

"We see things going from bad to worse every day. All we need is security and peace but I do not see this happening," said Abbas Hussein, 32, a carpenter. "I hope there will be a divine miracle so that the situation becomes stable."

A radical Sunni Muslim group, the Ansar al-Sunnah Army, claimed responsibility, saying a homicide bomber had carried out the strike.

Military officials in Iraq said Wednesday that shrapnel from the explosion included small ball bearings, which are often used in homicide bombings but are not usually part of shrapnel from rockets or mortars.

The attack sparked renewed concerns about the ability of U.S. troops and their Iraqi allies to secure elections Jan. 30. The military said they had expected an increase in violence as insurgents attempt to derail the vote for an assembly that will draft Iraq's new constitution.

"Insurgents, who have everything to lose, are desperate to create the perception that elections are not possible," said Gen. George W. Casey, the commander of multinational forces in Iraq. "We will not allow terrorist violence to stop progress toward elections."

In the immediate aftermath of Saddam Hussein's ouster in April 2003, U.S. commanders cited Mosul — with a population of 1.2 million some 220 miles north of Baghdad — as a success story. But armed opposition has mounted, especially since last month's successful U.S.-led operation to retake the insurgent-held town of Fallujah.

Many insurgents apparently moved to Mosul, where guerrillas launched a coordinated surprise attack in November against police stations. The municipal police force, estimated at over 6,000 officers, disintegrated; despite the success of U.S. troops a few days later in re-establishing control, only part of the police force has returned to work.