On Wednesday, Mosul's governor banned use of the five bridges that span the River Tigris in the city, and said anyone breaking the order would be shot. Residents said Iraq's third city was a virtual ghost town, with no one in the streets.
The order from Governor Duraid Kashmula was broadcast on local television and came into effect at 4am (0100 GMT).
Military investigators are still probing whether Tuesday's attack on a dining tent at the Marez US base in Mosul was a mortar strike, or possibly even a human bomber. It was the deadliest attack on the US military since the invasion last year.
In Mosul, people were afraid: "Students went to school but were told to go home," said Ahmad, 25, a car dealer who declined to give his surname. "People went to the shops, saw American troops in the streets, and went home."
Another resident described the city as shut down, adding that mosques and markets were virtually empty.
The US military said a 9pm to 5am curfew - imposed several weeks ago - remained in place, and confirmed it had stepped up operations in the search for suspects.
Mosul slid into chaos in recent
months with more than 200 dead
"We are conducting offensive operations to target specific objectives," Lieutenant Colonel Paul Hastings, a spokesman for US forces in the area, said.
Witnesses said US forces, backed by Iraqi National Guards, sealed off neighbourhoods in western and southeastern Mosul and raided homes. "They're looking in the areas that are known hotspots," one resident in the west of the city said.
Hastings said the source of the Marez attack was still unknown.
US officials initially said a number of rocket and mortar rounds were fired at a mess tent in southwest Mosul, but a group opposed to the presence of foreign troops in Iraq said a human bomber was behind the attack. A further 72 people were wounded.
"A suicide bomber has not been ruled out," a US army official in Washington said late on Tuesday.
"A suicide bomber has not been ruled out"
US army official
Giving the latest in a series of casualty updates, the military in Baghdad said 14 US soldiers, four American contractors and four Iraqi security force members were killed.
Fifty-one of the wounded were US military personnel. Of the 72 hurt, 43 were still being treated.
A spokesman in Mosul said it could take days before a clear picture emerged of what happened and who was killed.
"There's some tedious forensic work to be done. It could be a couple of days," said Captain Phil Ludvigson.
Mosul seems to be a city that has
fallen into lawlessness
Mosul has slid into chaos recently. In the past two months alone nearly 200 people have been found dead, most of them Iraqis, in a city of two million.
Mosul-based Iraqi journalist Abd Allah Ghafar said several streets leading out of the city were littered with bodies of Iraqi fighters, national guardsmen and those who were the target of score-settling revenge attacks.
"Mosul has never in its history witnessed such chaos, such lawlessness," he said by phone from the northern city.
He claimed foreign intelligence agents were behind some of the attacks in recent weeks.
"It is not likely that with the current security situation, elections will be held in Mosul," he said.