New details are emerging in the tragic and sudden death of Paul Walker and his longtime friend, Roger Rodas.

While investigators are trying to determine the circumstances surrounding the fatal single-car crash, there are a few theories trickling out as to what could have been the cause. Did the Porsche Carrera GT malfunction? Was there another vehicle involved? Or did the driver simply lose control?

Here is what we know as of Monday:

Speed Was a Factor

A Los Angeles County sheriff's statement said that "speed was a factor" in the tragic accident. However, a spokesperson for the Santa Clarita Sheriff's Station tells omg! the cause is still under investigation.

Were Paul and Roger Street Racing?

Sgt. Richard Cohen of the Santa Clarita Sheriff’s Station tells omg! they are looking into everything involved in the traffic collision, as they always do when speed is a factor. Cohen confirmed they are investigating a tip received regarding possible street racing, but have not been able to get a hold of the person who called in that report. Investigators are searching for video footage and any witnesses.

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Possible Car Malfunction

Insiders connected to Always Evolving — the auto shop Walker and Rodas owned where the vehicle was stored and maintained — allegedly tell TMZ they believe the crash happened as a result of mechanical failure.

Their sources cite a fluid trail, noticeable absence of skid marks until just before the point of impact, and the fact that the fire spread so quickly in the front of the car rather than in the rear where the engine is, as evidence to back up that theory.

"We are saddened to hear of this tragic accident," a spokesperson for Porsche tells omg!. "Of course, we will cooperate fully with the authorities on any post-accident investigation."

Again, investigators are looking into all possible causes.

Roger Rodas Was an Experienced Driver

Rodas, a 38-year-old financial adviser, knew more about maneuvering Porsches than most. As a pro racecar driver, he made his mark while taking part for many years in the Porsche Cup Cars series, finishing second in 2012, and this year competed in 10 GTS Series races, according to the Pirelli World Challenge organization.

The LA Times notes the street Roger and Paul crashed on is “a normally quiet road with a 45-mph speed limit," and that there is a sign warning drivers to slow down. It's a road Roger drove many times before. As loved ones search for answers, many are clinging onto this fact and don't believe Roger would have just lost control of the vehicle.

Autopsy on Hold

The Los Angeles Sheriff's Department will not release the names of the victims until an autopsy has been completed. The L.A. coroner's office confirms that examinations of the bodies have yet to be conducted, pending identification efforts. Dental records will need to be received to make positive identifications, which could take 2-3 days.

[Related: What Happens to 'Fast & Furious 7' Following Paul Walker's Death?]

Fans and 'Fast' Co-Stars Flock to Scene

Tyrese paid an emotional visit to the crash site, where he broke down in tears while leaving flowers for his "Fast" co-star and close friend on Sunday. He posted this heartfelt message on Instagram:

"I will keep your energy with me forever #AshesOfAnAngel fast family around the world we live you and we love you Paul is the heartbeat of this franchise and were gonna see to it that his energy and presence lives on forever #myhearthurtssobad."

Tyrese took with him debris from the accident.

Walker's on-screen love interest, Jordana Brewster, also visited the scene of the crash, but onlookers tell "omg! Insider" she didn't get out of the car.

Paul's Family Speaks Out

Paul Walker's family is mourning together.

"I'm just glad that every time I saw him, I told him I loved him and he would say the same thing to me," Paul Walker III told L.A.'s CBS2, choking back tears. Walker Sr. and his wife, Cheryl Walker, raised the 40-year-old actor and his four siblings in Sunland, California, where family and friends gathered on Sunday to grieve.

“You can’t beat friends at a time like this,” said Walker Sr. “His brothers are all taking it pretty hard."

Paul's father continued, “His heart was so open. I was proud of him every day of his life.”

Walker is also survived by his 15-year-old daughter, Meadow.