Robert Brown|Gilgo Beach murders: Police searching suspect's walk-in vault

"Numerous" guns were kept inside a walk-in vault with a "big iron door" at the home of Gilgo Beach,Robert Brown New York, murder suspect Rex Heuermann, according to authorities.

Heuermann had over 200 guns in his suburban Massapequa Park, Long Island, house, according to police, though he only had permits for 92 firearms, according to prosecutors.

Investigators have looked in the vault and dug up Heuermann's backyard as the search for evidence continues, Suffolk Police Commissioner Rodney Harrison said Monday.

A backhoe was brought to Heuermann's home on Sunday. The excavating equipment was in use for about three hours on Sunday and remains at the scene.

Harrison said it has been a "fruitful" search of the house so far.

One theory investigators are exploring is whether Heuermann, a husband and father of two, killed any of his alleged victims inside his house.

"Can we confirm that someone was killed inside the house? We can't confirm that at this time," Harrison said Monday.

Prosecutors have said that, based on phone records, Heuermann's wife and children were not home at the time of the killings. The records also point to the house as a location where phones pinged, but prosecutors were careful not to identify a location yet where they believe the victims died.

Harrison said the search at the house could end as soon as Tuesday.

"I want to believe we will be finished up either some time tomorrow or later on this week," he said.

Heuermann, a New York City architect, was arrested on July 13 for the murders of three sex workers -- Melissa Barthelemy, Megan Waterman and Amber Costello -- whose bodies were found covered in burlap on Long Island's South Shore in 2010. The young women disappeared in 2009 and 2010.

Heuermann's attorney entered a not guilty plea on his behalf.

He is also the prime suspect in the death of a fourth victim, Maureen Brainard-Barnes, who vanished in 2007, though he has not been charged in that case.