SAN JUAN, Puerto Rico—A new witness has come forward in the 2005 disappearance of American teenager Natalee Holloway in Aruba, and prosecutors said Tuesday they are seeking more evidence against the only remaining suspect.

A woman told Dutch police this month that Joran van der Sloot confessed to her years ago that he was involved in Holloway’s disappearance, according to Ann Angela, a spokeswoman for the Aruba Prosecutors’ Office.

But the Dutch Caribbean island’s chief prosecutor said authorities still lack proof they need to convict Van der Sloot, who has been arrested twice and released for lack of evidence.

“After three years of investigating, it is very, very difficult to find that evidence,” prosecutor Hans Mos told The Associated Press. “We have to be realistic.”

Holloway, an 18-year-old from Mountain Brook, Alabama, was last seen in May 2005 leaving a bar in the Aruban capital Oranjestad with Van der Sloot on the final night of a high school graduation trip to the island. Extensive searches have found no trace of her.