A passenger drags his luggage to his car after Royal Caribbean's Explorer of the Seas arrived back at Bayonne, New Jersey January 29, 2014.  [Photo/Agencies]

WASHINGTON - The US Centers for Disease Control and Prevention (CDC) said Friday its laboratory has confirmed that norovirus was the cause of the gastrointestinal illness that sickened nearly 700 passengers and crew aboard a cruise ship this week on a trip to the Caribbean.

"The CDC has been investigating the outbreak since last Sunday, but no particular source has been identified and it's quite possible a source won't be identified," the CDC said in a statement.

"This outbreak represents one of the largest norovirus outbreaks in the past 20 years," the agency added.

The cruise ship, Explorer of the Seas, which is operated by US -based Royal Caribbean, was forced to end its trip two days early and returned to home port in New Jersey Wednesday due to the outbreak.

According to the CDC, 634 of 3,071 passengers and 55 of 1,166 crew onboard have reported being ill during the eight-day voyage. The symptoms included vomiting and diarrhea.