MODESTO, Calif. -- Investigators used helicopters Monday in the continuing search for a pregnant Modesto mother who has been missing for almost six days.

Video

KCRA's Suzanne Phan Reports

FBI investigators were also interviewing family and friends in connection with the disappearance of 27-year-old Laci Peterson (pictured, right).

Peterson was reported missing Tuesday evening when her husband, Scott, returned home from a fishing trip in the Bay Area.

Investigators have combed the Peterson residence looking for clues.

Police said Scott Peterson told them his wife, who is eight months pregnant, planned to walk her dog in nearby Dry Creek Park and go grocery shopping when she vanished.

Monday morning, Modesto's Police Chief Roy Wasden described the latest investigation tactics.

"We're following every possible lead, including registered sex offenders, including family members, and looking at every potential. It's a frustrating case, with nothing concrete, no new suspects. So we're continuing to follow every potential lead as well as many called in leads," Wasden said.

Police say there have been 340 tips phoned to date, but nothing solid so far.

Police say they are checking vacant buildings and looking at surveillance video from stores that Laci may have been at before she disappeared.

The reward for helping find Peterson grew to $500,000 Saturday following a large donation from a man who asked to remain anonymous.

The man reportedly wrote a check for $375,000, quadrupling the reward for information leading to the safe return of Laci, who disappeared while walking her dog on the morning of Christmas Eve.

Authorities have used dogs, horses, helicopters and rafts in the search, but have come up empty.

Laci's husband (pictured, below right) said after returning from his fishing trip, he tried to call his wife's cell phone, but was not able to reach her. He said he found her purse in the house and her car in the driveway.

The dog was found earlier and returned by a neighbor. Police said the leash was still attached to the dog's collar.

Police concentrated their search Saturday on western Stanislaus County.

Officials at the volunteer search center at the Modesto Red Lion Inn say they need public help. Specifically, they need a computer and a printer, someone who can help make banners and signs, meals for volunteers, and office supplies like pens, tape, staples and copy paper.

A New Year's Eve candlelight vigil is planned for Peterson. The vigil will be held on Tuesday at 4:30 p.m. at East La Loma Park in Modesto.

To go to the Laci Peterson Web site, click here.