A highway bridge collapsed and hurled vehicles into the Mississippi River in central Minneapolis during evening rush hour on Wednesday, killing six people and injuring dozens, officials said.
Cars were crushed under huge slabs of concrete, flipped onto their roofs or thrown in the river as the 40-year-old bridge, packed with vehicles in nose-to-tail commuter traffic, collapsed with a thunderous roar.
The U.S. Department of Homeland Security in Washington said there was no indication of terrorism in the disaster.
"It will be a very tragic night when it is over," Minneapolis mayor R.T. Ryback told reporters. "Obviously this is a catastrophe of historic proportions," added Minnesota Gov. Tim Pawlenty.
The 500-foot (150-metre) span of the steel and concrete bridge, which carried an eight-lane highway and was under repair, buckled and fell into the river and onto concrete embankments at about 6:05 p.m. local time.
The span had stood about 65 feet (20 metres) above the river.
The local television station KARE said 58 people had been treated in nearby hospitals. Dr. Joseph Clifton told reporters that his hospital, Hennepin County Medical Center, had taken in 22 injured, six of them in critical condition.
One man was dead on arrival, having drowned. "Most were blunt-type injuries, in the face and extremities," Clifton said, adding many suffered internal injuries. More patients, and deaths, were expected, he said.
"I saw them carrying up a body -- I don't know if he was alive or dead," said Andy Schwich, who arrived at the scene on his bicycle a few minutes after the collapse.
A truck was exploding in fireballs, he said, and there were numerous cars either on the remnants of the bridge or in the river. The cause of the disaster was not yet known, state police said.
Gov. Pawlenty said the bridge, which was built in 1967, was inspected in 2005 and 2006 and the Minnesota Department of Transportation found no structural defects. Work was being done on the concrete surface, signs and guardrails.
Ryback said some 50 cars involved in the collapse had been searched by rescue workers. Rescue boats and divers searched the water on a hot evening as thunderstorms threatened the area.
A freight train was passing on a track running under the bridge when it collapsed, and the train was cut in two, WCCO television reported.
Five miles (8 km) of the Mississippi on either side of the collapsed bridge has been shut to river traffic, a spokesman for the U.S. Coast Guard in St. Louis, Missouri, said. The river, the longest in the United States, is a major transportation route.
Witnesses said they heard a rumbling sound as the bridge collapsed. "First I heard this huge roar," Leone Carstens, a nearby resident, told the Minneapolis Star Tribune. "I was at my computer. Initially I thought, Wow was that an airplane?"
Steel pilings stuck out at angles from the riverbed, huge chunks of concrete appeared to be floating in the debris-strewn water, and plumes of smoke rose from the site, a Reuters eyewitness said.
One witness said she saw people swimming in the water seeking safety and half-submerged vehicles. Injured survivors were led or carried up the embankment.
The state transport agency said 200,000 cars a day use the steel arch bridge.
In a similar disaster in June 1983, three people were killed when a section of bridge collapsed over a river on the I-95, the major U.S. East Coast highway, in Connecticut.