The lead investigator is positive they will eventually crack this case.
ORANJESTAD, Aruba (AP) -- Police have drained a pond, picked through landfill, scoured beaches and sent strands of blond hair overseas for genetic testing -- often chasing false leads in the investigation into U.S. teenager Natalee Holloway's disappearance.
The mystery baffles and frustrates investigators more than two months after the 18-year-old Alabama honors student was last seen leaving a nightclub with three young men.
"We thought we could solve this in a couple of days, but it turned out to be very difficult," said Lt. Roy Tromp, the lead investigator. "In my 30 years as a police officer, I've never had a case like this."
The most recent leads suggested something grisly happened to Holloway, but investigators say they have no proof she is dead.
The strands of hair were found on duct tape that washed up on Aruba's northern shore. DNA testing by the FBI determined the hair didn't come from Holloway.
A volunteer rescue group from Texas searched a landfill for a fifth day Wednesday, pursuing a tip from a witness who claimed to have seen men dumping a female body two days after Holloway disappeared. They've found nothing so far.
Last week, Aruban authorities almost completely drained a pond where a witness said he saw Dutch teenager Joran van der Sloot and two brothers, Satish and Deepak Kalpoe -- the three who left the nightclub with Holloway -- driving the night she vanished. The pond yielded up nothing.
Van der Sloot, 17, is the only person still detained in the case. The Kalpoe brothers, of Suriname, and four other men, including van der Sloot's father, were detained in June and released for lack of evidence.
The head investigator said he is convinced that van der Sloot was involved in Holloway's May 30 disappearance. Tromp said the teenager, who has maintained his innocence, has changed his account of what happened that night more than 10 times.
Three behavior specialists from the Netherlands questioned van der Sloot for a second day Wednesday, hoping to glean clues from his body language or version of events. An FBI observer was also present.
No one has been charged in the case. Searches throughout the Dutch Caribbean island have produced no trace of Holloway.
"We are still fighting. We are still positive we are going to break this thing," said Tromp.
But how long could it take?