FBI Special Agent Richard Wolf told The Associated Press that the U.S. agency is conducting interviews and giving other assistance to Aruba, which is taking the lead in the investigation. He declined to provide details or say who was being interviewed.
Gardner was reported missing Aug. 2 by traveling companion Gary V. Giordano, who is also from Maryland. The 50-year-old man told police that Gardner disappeared while they were snorkeling off the southern tip of the Caribbean island.
Aruban Solicitor General Taco Stein said earlier that police had detained Giordano because of seeming inconsistencies in his version of events. And on Thursday he said that if Gardner had drowned where Giordano had said, "We think that her body would have surfaced by now.
"We know the spot where he said she must have drowned. We have searched there extensively and in view of the weather situation we are almost certain that the body would have come to the surface," Stein said.
He said it was also possible that Giordano had told police to search in the wrong spot.
He said there were no unusual currents Aug. 2, when Gardner disappeared. "The water was very calm and clear ... The search took place all the way to Venezuela."
Giordano's Aruban lawyer, Michael Lopez, insists his client is innocent and is being unjustly detained. "There is no concrete or direct indication that our client might be involved in any illicit act concerning his friend."
On Thursday, Stein and prosecutors said they will soon send an official request for legal assistance to the U.S. Justice Department. They also urged people to come forward who may have seen the missing American woman and Giordano on the Dutch Caribbean island.
Prosecutors expect to release a photo of Giordano to aid potential witnesses, an unusual step in the Dutch legal system.
A statement issued by prosecutors Thursday evening said Giordano "no longer cooperates with the investigation and has decided not to answer any more questions."
While Gardner's tattoos may have been wild and rowdy, a close friend described her as a kind, quiet and trusting woman who sought solace in a tropical getaway with a man who is now suspected in her disappearance, a close friend said.
The 35-year-old divorced woman accepted Giordano's offer of a trip to Aruba despite earlier backing out of a cruise with the man she'd been friends with for over a year, Gardner's friend and part-time roommate Christina Jones said Wednesday.
"She's a very trusting person," Jones said at her hair salon in downtown Frederick, Maryland. "She's a hardworking, loyal, standup female who has never said anything bad about anyone."
Gardner's mother, Andrea Colson, said the family is hoping for the best outcome, praying Robyn will be with them soon and grateful to Aruban authorities and volunteers.
"I hope you understand that we are being quiet as to not jeopardize the investigation," Colson said in a statement released by the Natalee Holloway Resource Center, a Virginia-based missing persons group named for a young Alabama woman who vanished on Aruba in 2005.
Photos on a "help find Robyn" Facebook page feature Gardner's body ink, including a jungle-print design on her left shoulder and a Latin phrase tattooed over her right ribcage. Translated, it reads, "If I can't move the earth then I'll raise hell."
But the pretty, 5-foot-5-inch blonde was more reserved than the pictures suggest, according to Jones and to Gardner's on-again-off-again boyfriend, Richard Forester of Rockville.
At Forester's home, where she often stayed, she took care of two cats, Kobe and Toonsy, and relaxed by watching travel and food shows on TV, he said.
She also enjoyed watching the "Real Housewives of New Jersey" and the "Real Housewives of New York," Forester said.
"She just liked the drama of it," he said.
Forester said in an interview on NBC's "Today" show Thursday that Gardner posted a message on his Facebook wall about 2 a.m. on Aug. 2 saying "this sucks," but with no explanation. He said he emailed her throughout the day, and she responded that afternoon to say that she loved him and that they would sort things out when she returned.
Jones told the CBS "Early Show" that Giordano was a friend, not a boyfriend, of Gardner's and that the two had a "a bit of a roller-coaster" relationship. "Good one day, not so great the next," she said, adding she had only heard about Giordano through Gardner and never met the man herself.
A jogger and tennis player, Gardner also enjoyed traveling, whether to Florida, where her parents live, Las Vegas or Rehoboth Beach and Dewey Beach in Delaware, Forester said.
Gardner graduated from South Carroll High School in Sykesville in 1994. Forester said she had taken some college classes but didn't have a degree.
She was recently laid off from a dentist's office where she had worked scheduling patients, Forester said. She had also done some modeling, he said.
Giordano told police Gardner disappeared while snorkeling, but Forester, Jones and Gardner's brother Andrew Colson all said she wasn't usually that adventurous.
"Robyn's more of a lay-by-the-pool kind of gal," Jones said.
The prosecutor would not disclose their specific doubts, fearing it could undermine the investigation, but one of them is fairly basic: So far, there are no witnesses who saw the pair go snorkeling.
Giordano called police in Aruba around 6:30 p.m. that day to say he and Gardner got separated while snorkeling and she never made it back to shore. Video and photos obtained by the AP show him assisting police in a helicopter and boat search of the Dutch island's coastline.
Stein said police have the woman's computer, which they are considering sending to the Netherlands for analysis, and the couple's rental car, but no physical evidence of a crime.
The still-unsolved disappearance of Alabama teenager Natalee Holloway during a high school class trip in 2005, put Aruba in an unwelcome spotlight. Her remains were never found and the main suspect, Joran van der Sloot, is in jail in Peru on charges of killing a 21-year-old woman there last May.
Holloway's mother, Beth Holloway, said Gardner's disappearance resonated with her. Losing a loved one in a foreign country with a different language and an unfamiliar judicial system is even more difficult.
"It's hard to know who to be connected to, the right resources, and how to make those resources go to work for your missing loved one," she said in an interview from New York.
"I know the challenges ahead of them are great," she said of Gardner's family.
Jones described Gardner on Facebook as her "kindred soul." She said friends and family members remain hopeful she will turn up. Meanwhile, she said, the family wishes journalists would respect their privacy and let investigators do their work.
Associated Press writers David Dishneau in Frederick, Maryland, Jessica Gresko in Washington and Ben Fox in San Juan, Puerto Rico, contributed to this report.