ORANJESTAD, Aruba (AP) – They have drained a pond, picked through a landfill, scoured beaches and sent strands of blond hair overseas for genetic testing – often chasing false leads in the investigation into US teenager Natalee Holloway’s disappearance.

The mystery baffles and frustrates investigators more than two months after the 18 year-old Alabama honours 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 police 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 yesterday, 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 Surinamese brothers – the three who left the nightclub with Holloway – driving the night she vanished. The pond turned up nothing.

Van der Sloot, 17, is the only person still detained in the case. The Surinamese brothers and four other men, including van der Sloot’s father, were detained in June and later released for lack of evidence.

Tromp said he is convinced van der Sloot was involved in Holloway’s May 30 disappearance. He said the Dutch teenager, who has maintained his innocence, has changed his account of what happened that night more than 10 times.

Three behaviour specialists from Holland questioned van der Sloot for the second day yesterday, 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. Massive 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?

“I don’t have a crystal ball,” he said. “And as the days pass by, it’s becoming more difficult.”

Many Arubans, who say they take pride in their efficient police force, have become resigned to the possibility that there may never be a resolution.

“If she is alive, she isn’t here, and if she’s dead, they are not going to find her,” said Istela Inesia, 38, who runs a small deli.

American lawyers and police experts, in a stream of appearances on US cable television shows, have suggested Aruban authorities have bungled the investigation. Some have suggested there was a cover-up because van der Sloot’s father, Paul van der Sloot, is a Dutch judge in training.

Aruban police and citizens bristle at such speculation.

“Not Joran, not Paul, not anybody has had special treatment in this case,” Tromp said.

The missing teen’s mother, Beth Holloway Twitty, has kept the case in the spotlight with frequent media appearances, vowing not to let the investigation go cold.

The 44 year-old speech pathologist left Aruba last weekend for the first time since her daughter vanished, going home to Birmingham, Alabama.

But she was returning yesterday, determined to step up pressure on authorities.

“I’ll be here at least another month, unless we find Natalee sooner,” Holloway Twitty said.