Three people are in critical condition after a Twin Cities medical helicopter crashed near Alexandria, Minn., early Saturday, authorities said.

The North Memorial Medical Center helicopter went down about 2 a.m. Saturday on the east side of Lake Winona while on its way to the Alexandria airport. Three crew members, including a pilot and two medical personnel, were taken to Douglas County Hospital before being transferred back to North Memorial in Robbinsdale, according to the Douglas County Sheriff's Office.

North Memorial officials said there were no patients on board.

"I am extremely proud of our team members for their response at the site," said J. Kevin Croston, CEO of North Memorial Health Care. "Our immediate concern is with our crew members who are now being treated at North Memorial Medical Center. We are glad they are here. There is no better place to receive care."

The chopper appeared to go down on the northern perimeter of the airport, short of the runway, according to police scanner traffic. The airport was temporarily closed to all traffic in the aftermath of the crash.

Investigators have been called in to determine what caused the crash. The Federal Aviation Administration and the National Transportation Safety Board have been notified, a hospital spokeswoman said.

North Memorial owns and operates eight Agusta 109 helicopters, the fastest civilian helicopter on the market, which can reach speeds up to 180 miles per hour, according to the company's website. Air Care teams respond 24/7 to medical flight transportation service calls throughout Minnesota, areas of Wisconsin, Iowa and the Dakotas.

From 2011 to 2013, there were seven air ambulance accidents across the nation, resulting in 19 fatalities, according to the NTSB. By 2014, the FAA moved to increase safety measures on medical helicopters by requiring that pilots wear night vision goggles.

Weather conditions and cloud cover are major factors in these types of crashes, said Gary C. Robb, an attorney based in Kansas City, Mo., who specializes in helicopter crashes.

"They're occurring with alarming frequency," he said. "This is a real human tragedy. These are people who dedicate their lives to saving others, and that's what makes this so sad."