Two sightseeing planes carrying cruise ship tourists collided on Monday, leaving four passengers dead and ten wounded. Two people remain missing.

Federal investigators are expected to arrive in Ketchikan, a city south of Juneau, on Tuesday to try and determine what led the planes to collide in midair. In an email to the Associated Press, Federal Aviation Administration spokesman Allen Kenitzer reported that the collision was caused by unknown circumstances.

Both planes were carrying passengers from the Royal Princess cruise ship. One plane, a single-engine de Havilland Otter DHC-3 operated by Taquan Air, was carrying eleven people when it went down over the Tongass National Forest. Ten people were taken to a nearby hospital. Marty West, spokeswoman for PeaceHealth Ketchikan Medical Center, reported all patients to be in fair or good condition.

Three of the five people on board the second plane, a single-engine de Havilland DHC-2 Beaver, perished as a result of the collision. It has yet to be determined which plane the fourth victim was on board.

Emergency teams and locals helped federal and state agencies to rescue the stranded passengers after the crash.

“It’s been a long day and the crews have been working really hard to rescue people and recover the deceased,” said Deanna Thomas, who works as a spokeswoman for the Ketchikan Gateway Borough, the local government.

Taquan Air, the company who operated the first sightseeing plane, has suspended all operations while federal investigators work to determine the cause of the collision.

The Royal Princess cruise ship left Vancouver, British Columbia on May 11. It was scheduled to arrive in Anchorage on Saturday but Cindy Cicchetti, a passenger on board the cruise ship, said the ship was not on schedule to arrive in Anchorage and passengers were unsure how the accident will impact the remainder of the trip.