A medical helicopter crashed in a heavily wooded area in northern Wisconsin, killing all three people on board, authorities said Friday.

Search and rescue crews located the Ascension Health Spirit helicopter early Friday, according to Oneida County Sheriff's Chief Deputy Dan Hess. The wreckage was found near Hazelhurst, a small town about 150 miles (241 kilometers) northwest of Green Bay.

The three people killed were all flight crew members, Oneida County Medical Examiner Larry Mathein told Wausau television station WAOW-TV. No patients were on board.

The victims' identities have not been released, pending notification of their family members. The three are believed to have died on impact, Mathein said. The body of the pilot is being sent for an autopsy, which is standard protocol, according to the medical examiner.

Hess said didn't know if the three victims were all Ascension employees. He referred questions from The Associated Press to Ascension Health, which didn't immediately respond to calls and emails Friday morning. Mathein also didn't return messages from the AP.

Authorities said the last known contact with the helicopter was at 10:55 p.m. Thursday. Dispatchers received a call that the chopper was missing 25 minutes later, and a search was launched.

A witness who drove search crews to the scene on an amphibious all-terrain vehicle said the chopper went down in a heavily wooded area with high ridges and some swamps.

The National Transportation Safety Board will lead the investigation. At least eight agencies were involved in the search.