Tim Dolan, Minneapolis police chief, said that based on the number of vehicles thought to be on the Interstate 35W bridge at the time, there were 20 to 30 people unaccounted for, and more bodies were likely to be recovered.
"There are cars in the water. A dozen cars are visible. We don't know what's underneath. We haven't removed anything from the river. We're treating it like a crime scene," a police official said.
The bridge was undergoing repair work when it broke into several huge sections sending vehicles, concrete and twisted metal crashing into the river.
Several workers who had been doing resurfacing work that closed some of the span's eight lanes were among those who plunged into the river when the bridge fell about five stories.
Pawlenty said work "relating to concrete repair and rehabilitation and replacement, guard rail replacement, righting replacement and work on the joints," had been taking place.
The 40-year-old steel arch bridge was last inspected in 2006 and no structural problems were found, Pawlenty said.
A 2001 report by the state transport department concluded that the bridge "should not have any problems with fatigue cracking in the foreseeable future" and recommended not to replace the bridge "prematurely".
But it also pointed to problems with corrosion in the bridge's steel beams, "poor welding," and said it was designed under 1961 regulations that have since been rewritten with stricter rules.
RT Rybak, Minneapolis's mayor, said it was too early to pinpoint a cause for the collapse.
A spokesman for the US department of homeland security said there were "no indications of a nexus to terrorism at this time".