The investigation into the disappearance of 15-year-old Gabrielle Swainson has expanded into one of the Richland County Sheriff’s Department’s largest and most intensive searches in years.

More than 40 county and federal law agents are now on the case full time, Sheriff Leon Lott said Sunday night.

The agents include a behavioral scientist from the FBI’s famed psychological unit at Quantico, Va., Lott said.

Other FBI agents from out-of-state are in town working the case, along with local federal agents, Lott said.

Lott said late last week that he was asking for FBI help in the case.

But his Sunday statements were the first time he has said just how many resources the FBI is devoting to the missing teen, who lived in the North Crossing neighborhood, less than a mile from the Village at Sandhill mall.

“This is a major case for them,” Lott said.

Lott’s crime lab, reportedly one of the state’s best, has also had analysts and scientists working full time on the case, he said.

“We are on this 24/7 and have a command post set up coordinating things,” he said.

So many detectives are working on Gabrielle’s disappearance full time that early Aug. 19, after a security guard was shot and killed at a local nightclub, a detective supervisor who wouldn’t ordinarily work a killing was put on that case, Lott said.

Also, about 50 officials from his department and other departments are also on a standby status to swing into action if needed.

Four large black Suburban SUVs driven by FBI agents were seen at the Swainson house late Sunday afternoon in the 200 block of Tamara Way. Agents got in the vehicles and drove off.

Gabrielle, who should have started her sophomore year at Ridge View High School on Thursday, was last seen at her house in the early morning hours of Saturday, Aug. 18. At that time, she was sleeping in her bed when her mother, Elvia Swainson, left the house for a few hours to catch up on some work at her nearby office.

Gabrielle is believed to have been wearing her pajamas when she disappeared. The only known possession she had with her was her iPhone. Lott has declined comment on what, if any information, police have gotten from electronic queries to that iPhone. With modern cellphones, it is possible to triangulate their location, as well as get information about calls and text messages made to and from them.

A major reason that so many law officers are on the case is that it was completely out of character for Gabrielle to disappear. A good student, she was studying guitar and voice, went to church regularly with her mother and was a member of the Ridge View junior varsity cheerleading squad.

Although law officers fear an abduction, Lott said Sunday night, “We remain hopeful that we will find Gabbiee alive.”

The National Center for Missing and Exploited Children has also sent a consultant to work with Lott’s department.

Swainson family members could not be reached for comment.

Reach Monk at (803) 771-8344.