Moments after Steven Williams slammed his car into a Tarzana office building near the 101 Freeway over the weekend, paramedics and police arrived to help.

Lucid and talkative, Williams, 42, told them he had been alone in the now-mangled four-door sedan. Rescue crews then whisked him away to a local hospital.

Paramedics wrapped up the scene, officers took a report and a tow-truck driver hauled away the demolished car.

But, based on what the critically injured Williams said, nobody bothered to look inside the car. His mother, Shirley Williams, 72, lay crumpled in the passenger seat, still in her seat belt.

Everyone overlooked the 5-foot-3-inch, 145-pound woman hidden under a billowing air bag who died minutes after the crash, coroner's officials said.

On Monday, law enforcement officials were trying to figure out how those at the scene of the 10 a.m. Saturday crash on Tampa Avenue and the Ventura Freeway didn't see Williams, and why it took until Sunday afternoon to find her -- still in the front seat at a tow-truck yard.

``It just escapes your ability to comprehend,'' said Los Angeles Police Department Deputy Chief Michel Moore, who is overseeing the investigation. ``We are all looking at this and trying to understand what happened.''

Investigators didn't realize Shirley Williams was in the gold 2003 Mercury Grand Marquis until an official at Northridge Hospital Medical Center, where her son was admitted, called police to say he hadn't been driving alone.

A local hospital official said the mother and son from Paso Robles had been visiting family in Long Beach and the surrounding area on a holiday vacation. It is unclear why they got off the freeway at Tampa, or how the car ended up slamming into a bank across from the off-ramp, police said.

The force of the crash ripped open a gaping hole in the building and seriously injured Steven Williams. He was listed in critical condition Monday at the Northridge hospital.

``It looks like she was overlooked and nobody bothered to look in the vehicle,'' said Lt. Fred Corral of the Los Angeles County Coroner's Office. ``It's unfortunate they did not look in the vehicle. That's the first thing you do in a traffic accident.''

Los Angeles Police Department detectives have launched an investigation into why Shirley Williams was never spotted by all of the traffic officers, paramedics and the city building and safety inspectors who responded.

It's still unclear how many people were at the scene, Moore said. But after hospital officials tipped off police Sunday about a possible second person in the car, coroner's investigators and police descended on Howard Summer's Towing in Canoga Park and made the grim discovery.

A preliminary autopsy showed Williams died of multiple blunt-force injuries consistent with those caused in car crashes. She was pronounced dead at the tow company at 3:35 p.m. Sunday, more than 24 hours after the crash, although the coroner said she likely died minutes after the crash, Corral said.

There were no skid marks on the street and police were looking to see whether Williams' brakes malfunctioned, a source close to the investigation said.

West Valley Division detectives are talking to all fire, police and even building inspectors who were on the scene and consider the case an accident, not a homicide, Moore said.

He stressed that the investigation is in the preliminary stages and said police are trying to determine who spoke with Williams at the scene and why nobody looked in the car.

He noted that Williams was coherent enough to authorize somebody at the scene to tow the car, adding, ``We are scratching our heads as to how this could have happened.''