One of the "persons of interest" police are questioning as they probe who started a Southern California wildfire that killed four firefighters and charred more than 60 square miles of brushland is a convicted arsonist, FOX News has learned.

The fire was fully contained Tuesday as the investigation into who set the blaze moved into high gear. Authorities also picked over the spot where the blaze started last Thursday, at the base of a slope in Cabazon, west of Palm Springs. The fire spread quickly as fierce winds blew through the region. Before firefighters contained it Monday, the blaze scorched 40,200 acres and destroyed 34 homes.

At the crime scene Tuesday, investigators planted blue, red and yellow flags attached to wire stakes to mark the locations of possible evidence. Part of the charred hillside was marked by a grid of pegs and string.

Riverside County Sheriff's Department spokesman Sgt. Earl Quintana confirmed to FOX News that three people — two young men and a young woman — are considered "persons of interest" in connection with their investigation of the Esperanza fire that critically injured a fifth firefighter. Investigators swooped down Monday on a home only one-quarter mile from where the deadly fire was started last week and took them into custody for questioning. They were later released but questioning continued Tuesday.

One of the persons of interest, convicted arsonist William Thomas Hutson, told FOX News on Tuesday that authorities investigating the fire took his computer, "but over nothing," adding that, "they can come back if they want. They ain't gonna find nothin'."

He also said authorities didn't ask him anything when they confiscated his computer and that he took a polygraph test and passed.

James Crowell, an agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives, who is leading the investigation, said authorities interviewed more than one previously convicted arsonist who live in the area where the blaze started. In California, convicted arsonists must register with the county and provide their address.

Residents said they saw two young men leaving the area where the fire began last week.

"We're trying to work through the leads that we have, and going through the process," Riverside County Sheriff Bob Doyle said.

Doyle cautioned that investigators would be interviewing a number of people in the case. Quintana said there have been no arrests. The reward for information topped $500,000.

Hutson confessed to Vidor, Texas, police that he set a mobile home on fire in July of 1997 because he believed the new residents were gay. Hutson, who was convicted of arson and burglary and sentenced to 10 years in jail, was a former volunteer fire-fighter trainee in Vidor. He was released after serving 6 years in jail.

Hutson later told a newspaper reporter that the confession to the arson was coerced and claimed to have confessed to the arson so he wouldn't be charged with a hate crime. He also has on his extensive arrest record crimes for fraud, possession of controlled substance, and receiving stolen property.

Investigators entered the home just after noon PT (3 p.m. ET) Monday, and conducted an extensive search for evidence, including dusting for fingerprints, officials said.

Cabazon resident Robert Dunham told the Associated Press that in addition to the residents, sheriff's deputies took three bags of material from the home. The area is a quarter-mile from where the Esperanza fire started.

Another neighbor told FOX News one of the men taken into custody and placed in the back of a sheriff's patrol car was named "Jason," and described him as "a skin head."

Other nearby residents described the three young people as "troublemakers."

Investigators, including the FBI, have been searching since last Thursday for the arsonists responsible for starting the wildfire that killed four firefighters who were trapped when a wall of flames engulfed their engine while they were trying to protect a nearby home.

Officials immediately declared the fire the work of arsonists, and said that the firefighters' deaths was an act of murder.

More than 225 tips have been received by th Riverside County Sheriff's Department regarding the arson investigation, Doyle said.

Cabazon residents have told investigators that they saw two young men leaving the fire's ignition point.

Since it started around 1 a.m. Thursday, the fire burned 63 square miles, or 40,200 acres, northwest of Palm Springs.

Pablo Cerda, 23, the firefighter who was critically injured with burns over more than 90 percent of his body, underwent surgery Friday to remove damaged skin. While his condition was not upgraded Monday, he remained in critical condition Tuesday.

"Every day, it's just wait and see," said Tomas Patlan, a Forest Service spokesman. "They're trying to make sure he's breathing on his own, make sure there's no infections. Those are goals, one battle at a time."

Vigils were held at several Southern California churches and fire stations for Cerda and to pray for the families of the fallen firefighters. Killed were Mark Loutzenhiser, 43, of Idyllwild; Jason McKay, 27, of Phelan; Jess McLean, 27, of Beaumont; and Daniel Hoover-Najera, 20, of San Jacinto.

A memorial service for the dead firefighters was set for Sunday. Thirteen other firefighters suffered minor injuries battling the blaze.

More than 2,500 firefighters were involved in battling the blaze, which destroyed 34 homes, 20 out buildings and caused more than $8 million in property damage, to date, officials said.

FOX News' Jonathan Hunt, Nora Zimmett and The Associated Press contributed to this report.