FBI officials said Wednesday they believe a New York teenager who vanished in South Carolina seven years ago was held against her will for "several days" before she was murdered -- and appealed to the public for any information in her death. 

Brittanee Drexel, a 17-year-old high school junior from the Rochester, N.Y., area, disappeared in April 2009 while on spring break in Myrtle Beach, in a case that has baffled investigators. No arrests have been made and Drexel's remains have never been found. 

Authorities said Wednesday they have evidence to indicate Drexel was murdered in Georgetown County -- some 50 miles south of Myrtle Beach -- days after she disappeared from the popular seaside destination. They also offered a $25,000 reward for any information leading to the arrest and conviction of "those responsible for killing Brittanee Drexel."

The FBI and the Myrtle Beach Police Department held a joint press conference Wednesday afternoon in McClellanville, S.C., in Georgetown County -- the area where they say Drexel was killed. 

David Thomas, the FBI agent in charge of the case, said there are people in the community who know details that could put authorities "over the edge" in solving the case.

"We know there’s people that saw someone, heard something," Thomas said, noting it's a federal crime for anyone to withhold knowledge in the Drexel case.

"We do know that Brittanee was in this community for several days," Thomas said. "People came in contact with her, saw her, knew that she was here."

Thomas said it's not known whether Drexel traveled to Georgetown County willingly. But he said investigators believe that once there, she was held for several days against her will before she was killed.

Drexel went with friends and without her mom's permission to Myrtle Beach on April 23, 2009. A video camera at the Blue Water Resort captured her leaving the hotel on April 25 close to 9 p.m., police said. The footage is the last known sighting of her.

Investigators told FoxNews.com that a signal from Drexel’s cellphone was detected about 50 miles south of Myrtle Beach later that night – in a remote boat landing area near the South Santee River.

Police said Drexel, a petite blonde and avid soccer player, had left a group of male friends from Rochester at the Blue Water Resort to walk back to the Bar Harbor Hotel, where she was staying with other girls. After leaving the Blue Water hotel, Drexel texted her boyfriend back home to say she was headed for her hotel, but security video confirmed she never made it, according to police.

Myrtle Beach detectives previously said they believe Drexel was abducted as she walked the one-mile stretch of Ocean Boulevard between the two hotels. She may have been snatched off the street or willingly accepted a ride from the wrong person, police said.

Billboards promoting the $25,000 reward will go up along U.S. Route 17 in South Carolina beginning Wednesday.

The girl's mother, Dawn Drexel, addressed the community during the press conference with an emotional plea for information about her daughter.

"After seven long years of waiting and praying for the return of my daughter, we know she isn’t coming home alive," Drexel said. "Brittanee's life was stolen from her in a brutal fashion."

"She had her whole life ahead of her. She had a lot of dreams
and aspirations," she said. " They could have done this to anyone's child but they did it to mine. We need your help so we can bring Brittanee's remains home and lay her to rest."