The expanded search comes after investigators found the remains of at least nine people along the beaches of the South Shore.
"Please keep in mind that this is not an episode of CSI. This is an intensive long term investigation that includes the use of sophisticated technology and good old fashioned detective work," said Suffolk County Police Commissioner, Richard Dormer.
In addition to the searching, investigators are fanning out to interview more people.
Dr. Hackett is one of several people detectives have interviewed. The former head of Suffolk County Emergency Medical Services and former police surgeon told Eyewitness News that investigators have talked with him two or three times.
"It's beyond me, you know, the level of my belief," he said. "I can't even imagine it happening."
So far, police searching the south shore of Long Island have found the remains of nine people, and possibly a tenth. One of the victims was a toddler.
And with new spring growth making the dense foliage even more impossible to navigate, authorities have called in the FBI to fly overhead with sophisticated cameras that could see right through the brush.
Gilbert was last seen near Dr. Hackett's home last May. Gilbert's mother says she received a strange phone call from him the day after her daughter disappeared.
"He called me on the phone. He said he had a halfway house and that Shannon was there," she said. "She wanted to be off the streets and he was trying to help her."
Dr. Hackett denies calling Gilbert's mother, and police have not named any suspects in the case so far.
None of the nine bodies found so far have been identified as Gilbert.
The search in Suffolk County began in December, and with snow storms and frozen ground, it stretched on for three months. Police found the remains of four bodies last year, all of which have been identified. The five most recent sets of remains have yet to be identified. The earliest disappearance stretches back almost four years, to July of 2007.
Police say they've now ruled out a link with the murder of four prostitutes in Atlantic City.