The Centers for Disease Control and Prevention are investigating how more than 300 people have fallen ill on board a ship cruising the Caribbean.

CDC representatives boarded the Royal Caribbean's Explorer of the Seas Sunday after it docked at St. Thomas, U.S. Virgin Islands.

At least two CDC officials - an epidemiologist and an environmental health officer - were expected to do the investigation and evaluate the outbreak response on the cruise liner. The U.S. health agency did not immediately respond to a Sunday email and a call from The Associated Press seeking comment about the work aboard the ship.

In a Sunday statement, Beverly Nicholson-Doty, the U.S. Virgin Islands' tourism commissioner, said the territory was grateful for the CDC's "quick response" and St. Thomas was ready to welcome ship passengers cleared to disembark.

In all, 281 passengers and 22 crew members have reportedly fallen ill during the voyage, with most reporting vomiting and diarrhea. The CDC said it was not immediately clear what had caused the apparent outbreak. In response, the agency said that the ship's crew had stepped up its cleaning and disinfecting actions, encouraged passengers to report possible new cases, and prepared for new crew members to join the voyage midway through the journey.

Royal Caribbean International said in a statement Sunday that passengers were suffering from a gastrointestinal illness.

“Those affected by the short-lived illness have responded well to over-the-counter medication being administered onboard the ship,” the statement said. “We sincerely apologize for the disruption to our guests’ cruise vacation.”

At a stop in San Juan, Puerto Rico, “the ship underwent an extensive and thorough sanitizing to help prevent any illness from further affecting the cruise,” the statement added.

The ship had departed Cape Liberty, New Jersey on January 21 for a scheduled 10-day cruise to the Dutch Caribbean island of St. Maarten, which included scheduled stops in Haiti and Puerto Rico. There are a total of 3,050 passengers and 1,165 crew on board.

On Friday, an Explorer of the Seas passenger named Arnee Dodd tweeted that she had fallen ill aboard the ship and was quarantined with the other sick people. The Connecticut woman wrote that ship employees "put a lock down on food & are constantly cleaning everything."

It was not immediately clear how many passengers, if any, were still being quarantined on Sunday. The ship's next scheduled stop is St. Maarten.

This marks the second gastrointestinal outbreak on a cruise ship stopping at U.S. ports so far this year. An epidemiologist had to board a Norwegian Cruise Line ship in Miami on Jan. 19 after 130 passengers fell ill.

The last stricken Royal Caribbean ship was the Vision of the Seas, on board which 118 passengers fell ill last February, according to the CDC website.

The Associated Press contributed to this report.