Teams of US Navy and FBI divers have joined the search for victims of last week's bridge collapse in Minneapolis, stepping up recovery efforts.
The FBI will use an unmanned submarine to find evidence
The Navy divers will use sonar to scan for bodies, while the FBI team will carry out forensic investigations.
Minneapolis officials have called for a minute's silence on Tuesday evening to mark the moment the bridge fell.
Five people have been confirmed dead and eight missing in the disaster, but officials say those figures may rise.
About 100 people were injured when the eight-lane I-35W highway bridge collapsed into the Mississippi river at evening rush hour on 1 August.
'World's best divers'
The Navy divers entered the murky waters before dawn on Tuesday to survey the search site and returned later in the morning to clear debris from the site.
The Navy team - 16 divers and a five-member crew - are using above-ground oxygen tanks so they can spend longer underwater. They will also use heavy-duty equipment to cut through steel wreckage.
Minneapolis Police Captain Mike Martin told Associated Press news agency the Navy team were "the best divers in the world".
"Two in the morning, they dove into the water," he said.
Divers from the FBI's Underwater Search Evidence Response Team (USERT) will use an unmanned submarine with a robotic arm to look for any evidence that may help with the investigation.
Minneapolis city officials called for a minute's silence at 1805 (2305 GMT) in memory of the bridge collapse victims.
After the silence, bells were to toll at churches, Minneapolis City Hall and the city's court house.
On Monday, the US National Transportation Safety Board said the investigation into the causes of the bridge collapse may take as long as 18 months.
State officials have said they hope to have the bridge rebuilt by the end of 2008.