NEW YORK (Reuters) - A 17-year-old New York girl who vanished in 2009 while on spring break in a South Carolina beach town was likely held captive for several days before being killed, federal agents investigating the girl's disappearance said for the first time on Wednesday.

Rochester, New York resident Brittanee Drexel, who was last seen on surveillance camera leaving her waterside hotel in the tourist hotspot Myrtle Beach, had been treated as a missing person since she vanished seven years ago.

Drexel's case, which was featured on the Discovery Channel’s “Disappeared” television show, has some similarities with the highly publicized vanishing of Mississippi teenager Natalee Holloway, whose fate is still a mystery.

Drexel's remains have not been found, and no one has been arrested in connection with her disappearance. After hundreds of interview and leads, investigators said they believe Drexel died in McClellanville, South Carolina, the last place Drexel's cell phone signal was traced to. McClellanville is a fishing town about an hour's drive south down the coast from Myrtle Beach.

"It is very difficult but the public needs to know, the family needs to know," said U.S. Federal Bureau of Investigation special agent in charge David Thomas, who spoke at a news conference from the remote area, which was streamed live on the web.

Thomas declined to say how the FBI knew Drexel was dead or had been held captive. It was not clear whether the girl had chosen to travel to McClellanville from Myrtle Beach or if she was taken to the town against her will, he said.

Mother Dawn Drexel appealed on Wednesday to anyone who could help solve her daughter's case.

"After several long years of searching for my daughter, we know she isn't coming home alive," Drexel said during the news conference. "We need your help so we can find Brittanee's remains and lay her to rest."

A $25,000 reward is being offered for information leading to the capture and conviction of Drexel's killer or killers.

"We think we're at a point where one or two small pieces of information could put us over the edge," Thomas said.

In a similar case, Holloway vanished while on a high school graduation trip with friends to the Caribbean Island of Aruba in 2005. Despite strong public interest and the involvement of U.S. law enforcement, her case remains unsolved.

(Reporting by Laila Kearney; Editing by Bill Rigby)