Story highlights Sheriff's office says it is focusing on speed as the cause of Saturday's fiery crash

Walker family asks for charity donations instead of flowers

Walker's racing team partner was driving the Porsche

The actor was in the middle of producing the seventh "Fast & Furious" movie when he died

Investigators have been unable to find evidence of a second car in the accident that killed popular "Fast & Furious" actor Paul Walker on Saturday, a Los Angeles County Sheriff's spokesman said Monday.

The probe now centers on the speed of the 2005 Porsche Carrera GT, driven by Walker's racing team partner, the spokesman told CNN.

Earlier, Los Angeles County Sheriff's Sgt. Richard Cohen told CNN that authorities received a tip Sunday suggesting that another car was at the scene racing the Porsche when it slammed into a light pole and burst into flames.

But investigators have since ruled out the presence of a second vehicle and the theory that the Porsche was drag racing, a spokesman said Monday afternoon.

Speed was a factor in the crash, the Los Angeles County Sheriff's Office said. A 45 mph speed limit sign was attached to the light pole knocked down by the Porsche.

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The car, which sold for $450,000 when new, is a notoriously difficult vehicle to handle, even for professional drivers, according to Autoweek magazine. A top driver called it "scary," the magazine reported Sunday. It is powered by a V-10, 610-hp engine.

Jim Torp, a car enthusiast who was at the charity event that Walker attended before the wreck, said Walker was smiling as he got into the Porsche minutes earlier.

Photos: Paul Walker: Life in the fast lane Photos: Paul Walker: Life in the fast lane Paul Walker, a star of the "Fast & Furious" movie franchise, died in a car crash on November 30, 2013. He was 40. Here's a look at his career through the years. Hide Caption 1 of 15 Photos: Paul Walker: Life in the fast lane Walker with Tyrese Gibson, Michelle Rodriguez and Ludacris in "Furious 7," which had mostly completed filming when Walker died. Many fans got emotional upon seeing Walker onscreen in the movie, which earned $143 million its first weekend. Hide Caption 2 of 15 Photos: Paul Walker: Life in the fast lane Walker's role as Brian O'Conner in the "Fast & Furious" franchise (here with Vin Diesel in the sixth film) helped make the actor a star. Hide Caption 3 of 15 Photos: Paul Walker: Life in the fast lane Walker and Jordana Brewster -- who played his love interest, Mia, in the films -- in "Fast 5." Hide Caption 4 of 15 Photos: Paul Walker: Life in the fast lane Kevin Rankin and Walker appear in "Pawn Shop Chronicles," a 2013 film about a missing wedding ring that sets off a wild goose chase. Hide Caption 5 of 15 Photos: Paul Walker: Life in the fast lane Walker also had a starring role in the 2013 film "Hours," about a man trying to keep his daughter alive after Hurricane Katrina. Hide Caption 6 of 15 Photos: Paul Walker: Life in the fast lane The actor was part of a band of thieves in the 2010 movie "Takers." Hide Caption 7 of 15 Photos: Paul Walker: Life in the fast lane Brooklynn Proulx and Walker in the 2008 film "The Lazarus Project." Hide Caption 8 of 15 Photos: Paul Walker: Life in the fast lane Walker, left, and Barry Pepper were U.S. Marines in the 2006 drama "Flags of Our Fathers." Hide Caption 9 of 15 Photos: Paul Walker: Life in the fast lane Walker appears in the 2005 action film "Into the Blue," which also starred Jessica Alba. Hide Caption 10 of 15 Photos: Paul Walker: Life in the fast lane Tyrese Gibson took over for Diesel opposite Walker in 2003's "2 Fast 2 Furious." Hide Caption 11 of 15 Photos: Paul Walker: Life in the fast lane Walker appeared in 2001's "The Fast and the Furious," the first movie in the franchise. Hide Caption 12 of 15 Photos: Paul Walker: Life in the fast lane Walker, center, appeared in "Skulls" with Leslie Bibb and Joshua Jackson in 2000. Hide Caption 13 of 15 Photos: Paul Walker: Life in the fast lane Walker and Freddie Prinze Jr. co-starred in the popular 1999 teen film "She's All That." Hide Caption 14 of 15 Photos: Paul Walker: Life in the fast lane Walker's first major role was opposite Reese Witherspoon in the 1998 film "Pleasantville." Hide Caption 15 of 15

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Torp thought he heard a blast in the distance before the car slammed into a light pole, he said. "What the first explosion was, I don't know if their tire blew up, because it sounded like a tire blew on the car," Torp told CNN Monday.

Tire skid marks on the asphalt near the crash site, which indicate a car was doing doughnut spins, also are being looked at, Cohen said. It has not been concluded that they are related to the Walker wreck, he said. The street has a reputation for fast drivers, which spurred a crackdown by deputies two years ago, he said.

Torp told CNN that he looked closely at the skid marks and concluded they were left by a car with smaller tires.

Walker and Roger Rodas, who was believed to be driving, died in the wreck on Hercules Street, a wide business park road, in the community of Valencia inside the city of Santa Clarita, about 30 miles north of Hollywood, according to Walker's publicist and CNN affiliate KCAL.

The autopsies on the remains taken from the wreckage are scheduled for Tuesday, according to Lt. Fred Corral of the Los Angeles County coroner's office. Investigators are still waiting for dental X-rays for each man to help in making the official identifications, Corral said.

Stunned fans, a grieving father

Walker's family issued a written statement Monday expressing thanks for "the outpouring of love and goodwill from his many fans and friends."

They asked that instead of flowers, people make donations to his charity Reach Out Worldwide

"It's comforting for them to know that the son, brother, and father they love so much is equally adored, respected, and appreciated by so many," the statement said. "Paul founded the organization with the genuine desire to help others, and it's important to his family to keep his memory alive through ROWW."

No memorial service plans have been made, Walker's publicist said Monday.

Stunned by Walker's untimely death, fans, friends and family remembered what he meant to them and what he did for them.

His father remembered him as a loving son who wanted to take a hiatus from acting. A fellow actor said he had just celebrated Walker's 40th birthday. And a U.S. military veteran will forever be grateful for a touching act of generosity.

Day and night since the crash, legions of fans gathered near the charred roadside where Walker died.

Tyrese Gibson, Walker's co-star in several "Fast & Furious" movies, broke down as he laid a yellow flower at the site.

"My heart is hurting so bad no one can make me believe this is real," the singer and actor posted on Instagram . He also shared the duo's last text exchange

Paul Walker Sr. choked back tears as he remembered his son.

"His heart was so big," he told CNN affiliate KCAL . "I was proud of him every day of his life."

The actor told his father that he wanted to take a hiatus from acting to spend more time with his 15-year-old daughter, Meadow, the elder Walker said. "And then boom, he got another movie. He would say, 'I don't know what to do.'"

He said the actor's siblings are having an especially hard time grappling with the death.

"I'm just ... glad that every time I saw him, I told him I loved him," the father said. "And he would say the same thing to me."

Remarkable generosity

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Tales of Walker's philanthropy are not new. CNN confirmed one story from a decade ago when Walker noticed a young U.S. soldier shopping with his fiancee for a wedding ring in a Santa Barbara jewelry store.

"The groom was just back from duty in Iraq, and he was going to be deployed again soon and wanted to buy a wedding ring, but he said he just could not afford it," saleswoman Irene King told CNN. "I don't think the soldier realized how expensive those rings are, about $10,000."

The couple apparently did not know who Walker was, King said.

"Walker called the manager over and said, 'Put that girl's ring on my tab,'" she said. "Walker left all his billing info, and it was a done deal. The couple was stunned. She was thrilled and could not believe someone did this."

King called it "the most generous thing I have ever seen."

Future of franchise

At the time of his death, Walker was working on the seventh film of the franchise, due out next year.

It's unclear how the film's production might proceed. When Oliver Reed died in the middle of the production of "Gladiator," the rest of his scenes included a digitally produced image of his face on another actor's body, Tom O'Neil, editor of the show business website Goldderby.com, told CNN.

In some cases, other actors have filled in for co-stars who have passed away.

"We don't know what they'll do here, or even if they'll just say, "It may be tasteless to proceed at all because we can't be showing Paul Walker in a speeding car, defying death in a movie that ended up being the way he died,'" O'Neil said.

Questionable speed

The wreck took place about 3:30 p.m. Saturday just a few hundred yards from the shop owned by Rodas. Both men had attended a holiday toy drive for Walker's charity hosted at the shop that afternoon.

Antonio Holmes told the Santa Clarita Valley Signal newspaper that he was at the charity event when Walker and Rodas left for a ride in the Porsche.

"We all heard from our location," Holmes told the Signal. "It's a little difficult to know what it was. Someone called it in and said it was a vehicle fire. We all ran around and jumped in cars and grabbed fire extinguishers and immediately went to the vehicle. It was engulfed in flames. There was nothing. They were trapped. Employees, friends of the shop. We tried. We tried. We went through fire extinguishers."

A crowd of grieving fans, curious onlookers and media surrounded the crash site for hours, watching as investigators and firefighters worked to extract the bodies from the wreckage.

Walker and Rodas had planned Saturday as a day to help survivors of Typhoon Haiyan in the Philippines. The car shop website invited customers to a "Charity Toy Drive & Automotive Social Gathering."

"During the holiday season, many economically disadvantaged children from around the world are faced with the same dilemma year after year; the lacking of joy and cheer," the invitation read. "Our goal here is to be able to provide aid to these less fortunate children in hopes of helping them grow up to become confident, responsible and productive young adults."

Walker's charity is described as "a network of professionals with first responder skill-sets who augment local expertise when natural disasters strike in order to accelerate relief efforts."

Box office success

Walker's career began on the small screen, first with a commercial for Pampers when he was 2, and then with parts in shows such as "Highway to Heaven" and "Touched by an Angel."

His first few movie roles were as supporting characters in teen flicks, most notably in "Varsity Blues." But his career really took off when he was cast as undercover cop Brian O'Conner infiltrating a street-racing gang in 2001's "The Fast and the Furious."

The box-office success of the surprise summer hit yielded numerous sequels. And along with Vin Diesel, Walker was one of the franchise stalwarts. The six "Fast & Furious" films sold a total of $2.6 billion in tickets worldwide, according to BoxOfficeMojo.

Walker wasn't just a car enthusiast on the silver screen; off screen, the actor competed in the Redline Time Attack racing series.

On his verified Twitter account, Walker described himself as an "outdoorsman, ocean addict, adrenaline junkie ... and I do some acting on the side."

Walker also is the star of "Hours," an independent film scheduled to be released December 13 about a father struggling to keep his newborn infant alive in the aftermath of Hurricane Katrina in New Orleans.