Princess Cruises is investigating how two passengers tumbled off a cruise ship cabin balcony and into the Gulf of Mexico, trigging a four-hour rescue effort.

The 22-year-old man and 20-year-old woman fell 50 to 60 feet early Sunday from the woman's cabin. Julie Benson, spokeswoman for Princess Cruises, said it appeared to have been an accident.

The ship, the Grand Princess, was about 150 miles off the coast of Galveston, Texas when friends notified the crew around 1:30 a.m. that the two had fallen, Benson said.

The captain turned the ship around, and the crew used high-powered spotlights to scan the water and launched rescue boats to find them. One passenger was pulled into a rescue boat around 5:30 a.m., and the other was found about 30 minutes later, according to a statement by the cruise line. A Coast Guard helicopter crew, called in to help, spotted the male passenger.

The man and woman, whom the cruise line declined to name, were examined by the ship's medical staff. They sustained minor injuries, Benson said.

"Just falling 50 feet, can you do some serious damage and basically knock the wind out of you," said Coast Guard swimmer Micah Franklin, who participated in the rescue. "If you survive that part, then you've got, depending on your swimming ability, how long are you going to be able to survive in the seas."

The waves were swelling up to 6 feet, and the wind was gusting to up to 30 mph, Franklin told NBC's "Today" show Monday. He said he was impressed the two survived.

The rescued passengers were not married and did not come aboard together, the Houston Chronicle reported Monday. Both were cooperating with investigators, Benson said.

Benson said the railing around the cabin balcony is at least 4 feet high and meets all standard safety requirements.

"What we're looking at is how they went over," she said. "That's all part of the investigation."

The ship was on its way to Costa Maya on Mexico's coastline, Benson said. The Grand Princess was carrying 2,783 passengers on a seven-day tour of the western Caribbean.