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   Mount Vernon, WA (ABC) - Three people were sent to the hospital after a portion of an Interstate 5 highway bridge in Mount Vernon, Wash., collapsed Thursday, dumping three vehicles into the water.

   Two people rescued from the water were suffering from hypothermia, police said. Skagit Valley Hospital in Mount Vernon, Wash., was expecting two patients taken from the accident, according to Kari Ranten, a spokeswoman. She believed the third patient was transported to another hospital.

   Officials were looking into reports of an over-sized load "immediately" causing the collapse, said Travis Phelps of the Washington State Department of Transportation and Washington State Patrol.

   "We're looking at the cause being an over-sized, over-height vehicle, striking critical portions of this bridge, causing it to collapse," he said.

   The National Transportation Safety Board said they will send a team to investigate the collapse.

   The collapse occurred on the portion of Interstate 5 over the Skagit River, about two hours north of Seattle.

   "N/B and S/B lanes of I-5 Skagit River Bridge collapsed," Washington State Trooper Mark Francis posted on Twitter. "People and cars in water."

   The collapse occurred around 7 p.m. local time and the portion of the bridge that collapsed was four lanes wide, The Associated Press reported. The vehicles plunged about 40 feet from the bridge into the river and that set off a massive rescue operation.

   A damaged red car and a damaged pickup truck were visible in the water. Helicopter footage from ABC News affiliate KOMO-TV showed several rescue boats in the Skagit River with several ambulances waiting on the shore.

   Xavier Grospe, 62, who lives near the river, told The Associated Press he could see three cars partially submerged in the water with what appeared to be one person per vehicle, with drivers on top of vehicles or sitting on open window openings.

   "It doesn't look like anybody's in danger right now," Grospe said.

   The bridge was built in 1955, according to the AP, citing federal records.

   Clean up efforts will take several days to weeks, according to Phelps. The bridge sees 77,000 cars per day, and Phelps said they are expecting significant congestion until the bridge is fixed.

   "We inspect our bridge every two years. We're not going to let anybody drive on a bridge that is deemed unsafe," Phelps said.

   The bridge was last inspected in November 2012 and deemed safe, Washington State Department of Transportation spokesman Bart Treece told ABC News.