US Defence Secretary Donald Rumsfeld has come under pressure over the attack at a US base in the northern Iraqi city of Mosul that killed 22 people.
Soldiers had just sat down to eat when the attack occurred
Investigations so far suggest the blast was caused by a suicide bomber, the Pentagon chief told a news conference.
Journalists asked Mr Rumsfeld to explain how security could have been breached and troops left vulnerable.
The defence secretary said he was truly saddened by claims that he was failing in his duty to protect the troops.
But both Mr Rumsfeld and the Chairman of the US Joint Chiefs of Staff Gen Richard Myers looked embattled and weary as they entered the Pentagon briefing room, says the BBC's Rob Watson in Washington.
These are difficult times for Mr Rumsfeld to be fighting back against his critics, our correspondent says.
Part of Mosul is under curfew after the blast, which hit a packed dining hall.
Of those who died, 13 were US troops, five US civilian contractors, and four Iraqis.
In other developments:
- Two French reporters return home from Iraq after being held hostage for four months
- US construction firm Contrack International pulls out of a $325m (£170m) contract to rebuild Iraq's transport infrastructure, citing security fears
- The first group of civilians displaced by fighting in Falluja prepares to return home on Thursday.
Gen Myers told reporters "an improvised explosive device worn by an attacker" was the most likely cause.
"We have had a suicide bomber apparently strap something to his body... and go into a dining hall," he said.
He said further inquiries - in which the FBI is involved - would ask how an attacker gained access to the base.
Gen Myers did not say what evidence had led investigators to favour the suicide attack theory - initial reports said the explosion had been caused by a rocket or mortar.
If confirmed, it would be the first time a suicide bomber has managed to get inside a US military installation within Iraq, says our correspondent.
On Tuesday, President George W Bush vowed the attack would not derail Iraqi elections planned for next month.
Mr Rumsfeld said it was "unrealistic" to think insurgents would rest after the election.
"Looking for a peaceful Iraq after the elections would be a mistake," he said.
On Wednesday Mosul's governor banned the use of all five bridges into the city, as hundreds of US troops started sweeping the area to hunt for suspects.
City streets were deserted, with shops and even mosques closed, residents in the city of two million - and Iraq's third largest - said.
The attack has been claimed by militant group Ansar al-Sunna, thought to have links with leading Jordanian militant Abu Musab al-Zarqawi.
Mosul has experienced a wave of violence since last month's offensive on the former rebel stronghold of Falluja.