Federal investigators were heading to Alaska on Tuesday to determine why two sightseeing floatplanes collided in midair, killing at least four people and injuring 10 others.

Two people remained missing after the crash Monday near Ketchikan.

The planes, featuring pontoons that allow them to land on water, were flying cruise ship passengers on tours of Misty Fjords National Monument in the nearby Tongass National Forest, Princess Cruises said in a statement.

"There are two people that we're still searching for at this time," Coast Guard Petty Officer Jon-Paul Rios told ABC News on "Good Morning America." "Our crews are out there searching diligently alongside our partner agencies and some good Samaritans."

Ten people were being treated at Peace Health's Ketchikan Medical Center. One person was in critical condition initially, but all were later listed in fair or good condition, spokeswoman Marty West said.

Chris John, an incident commander with the Ketchikan Volunteer Rescue Squad, told the Anchorage Daily News that a de Havilland DHC-2 Beaver with five people aboard crashed on a steep rocky shoreline, ending up upside down and partially submerged in saltwater. The other plane, a de Havilland Otter DHC-3 with 11 aboard, crashed about a mile away on the other side of an inlet, he said.

The planes crashed under unknown circumstances, Federal Aviation Administration spokesman Allen Kenitzer said.

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

A spokesperson for Taquan Air, which operated the plane carrying 11 people, said the company is cancelling flights while federal authorities investigate the incident.

“At this time, we are in the midst of an active crisis response, and our focus is on assisting these passengers, the pilot, our staff, their families and loved ones, and first responders," Taquan said in a statement.

Weather conditions at the time of the crash included high overcast skies with 9 mph southeast winds.

The cruise ship Royal Princess had departed Vancouver last weekend for a planned seven-day cruise, ending in Anchorage.

"We are deeply saddened to report this news and our thoughts and prayers are with those who lost their lives and the families of those impacted by today’s accident," Princess Cruises said in a statement.

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In 2015, a floatplane crashed into a mountain northeast of Ketchikan, killing all nine people aboard. The pilot got disoriented in cloudy weather while his company pressured him to get back to a cruise ship before it departed, federal investigators determined.

In the eight years before the accident, Alaska recorded four fatal air-tour crashes involving cruise ship passengers, Robert Sumwalt, acting chairman of the National Transportation Safety Board, said at the time.

Contributing: The Associated Press