Milwaukee ? A medical transport plane carrying six members of an organ transplant team went down Monday afternoon in Lake Michigan shortly after the pilot signaled an emergency, authorities said. Rescue teams were searching for any survivors.

Nearly three dozen divers were looking near debris and an oil slick in about 20 feet of water, Milwaukee Fire Chief Doug Holton said.

The pilot of the Cessna Citation issued a distress signal within five minutes of taking off at 4 p.m. from General Mitchell International Airport in Milwaukee, Federal Aviation Administration spokesman Tony Molinaro said. The plane was headed for Willow Run Airport near Detroit, a 42-minute flight, Molinaro said.

The plane dropped off radar screens just after the pilot requested to return to the airport, and authorities notified the Coast Guard, Molinaro said.

Coast Guard searchers found what they believe was the plane about 4:20 p.m., about 20 minutes after being notified, Petty Officer David Warfel said. The debris was found six miles northeast of Milwaukee, Molinaro said.

There has been no sign of the four passengers or two crew members, Molinaro said.