After five weeks of digging, crews have found debris, but no human remains during an excavation of an abandoned well in Central California in a search for possible victims of two men known as the "Speed Freak Killers," the FBI said.

A team of FBI personnel and technical experts have gone more than 95 feet below grade -- or about four stories down -- as they remove and examine debris from the narrow, five-foot-wide well, FBI spokeswoman Gina Swankie said Monday.

More than a year ago, San Joaquin County officials found the remains of three bodies and a fetus not far from the current search site. Authorities thought the remains may have belonged to victims of Wesley Shermantine and Loren Herzog.

Two other bodies were found buried elsewhere around the same time. Four of the bodies have been identified as long-missing women suspected of being killed by Shermantine and Herzog.

Shermantine and Herzog were arrested in 1999 and convicted of several murders each.

Shermantine was sentenced to death after a jury convicted him of four murder charges.

Herzog was convicted of three murders and sentenced to 78 years-to-life prison, but the convictions were overturned by an appeals court that ruled his confession was illegally coerced. He later pleaded guilty to voluntary manslaughter and was paroled in 2010.

Herzog hanged himself in January 2012 after Shermantine began directing authorities to grave sites.

The current excavation work will continue until the historic bottom of the well has been reached, Swankie said. The FBI is leading the current excavation effort after San Joaquin County sheriff's officials requested the agency's help.