Federal agents yesterday arrested the co-owner of the car used by the suspects in the Washington area sniper case, while police in Washington state sought evidence that might link one of the principal suspects to a killing in Tacoma, Wash., eight months ago.

The action yesterday came as investigators probed whether bank robberies in the Washington area were committed by the sniper suspects to bankroll the operations. They also scrutinized a handgun recovered in their car for possible links to shootings elsewhere and the records of a Tacoma gun shop where the rifle allegedly used by the suspects was obtained.

The funeral for the 10th person fatally shot during the three-week spate of killing, bus driver Conrad E. Johnson, drew thousands of mourners and a cortege of 30 buses from as far away as Indiana, driven as a tribute to the victim by his peers.

The burial was conducted under sunny autumn skies that drew tens of thousands of people out to less somber pursuits throughout the Washington region, many simply rejoicing at the opportunity to frolic outdoors as the pervasive fear of a sniper's bullet dissipated. People flocked to shopping centers, played in parks, pumped gas with impunity and strolled without care.

The sniper task force headquarters in Rockville went virtually silent after weeks of intense activity. But even as most investigators paused to relax, others continued to gather evidence about the activities of the two suspects -- John Allen Muhammad, 41, and John Lee Malvo, 17 -- who were taken into custody as they slept in their car at a highway rest stop west of Frederick before dawn Thursday.

The man listed as the co-owner of that car, Nathaniel O. Osbourne, 26, was arrested without incident yesterday in Flint, Mich., by federal agents at what was believed to be the home of a friend. Osbourne, who is listed as co-owner of the car with Muhammad on a vehicle registration filed in New Jersey, was being held as a material witness in the sniper case.

He was being questioned in Flint, pending a bond hearing scheduled for today, and law enforcement sources said he was "fully cooperating."

FBI spokeswoman Debbie Weierman said he was not a suspect in the shootings, but "we believe he has valuable information about the sniper case."

The origins of Osbourne's acquaintance with Muhammad were unclear, but the blue 1990 Chevrolet Caprice was purchased in the name of both men last month and registered at the Camden, N.J., address of a restaurant where Osbourne worked.

Meanwhile, another of Muhammad's relationships came under scrutiny in connection with the Feb. 16 unsolved homicide of Keenya Cook in Tacoma.

Although relatives said Cook did not know Muhammad, the suspect was a close friend of her aunt and uncle's.

"We obtained information on Thursday that there was a connection between Muhammad and the Cook case," Jim Mattheis, spokesman for the Tacoma Police Department, said yesterday. "This is something we feel needs to be investigated. It appears there was some conflict between Muhammad and the aunt, so it's definitely something we're looking into."

Cook shared a Tacoma house with her aunt, Isa Nichols, who helped Muhammad launch an unsuccessful car repair business almost 10 years ago. But tensions developed between Nichols and Muhammad after she sided with Muhammad's wife when the two became estranged almost two years ago, a Nichols family member said yesterday.

Cook was alone in the house with her infant Feb. 16 when she answered the door and was shot in the face with a .45-caliber handgun, police said.

The family did not suspect Muhammad in the shooting but contacted police last week after learning that he was a suspect in the sniper case, Joseph Nichols, Cook's uncle, said yesterday.

Tacoma investigators are interested in a handgun allegedly recovered from Muhammad's car after Thursday's arrests. Detectives collecting evidence in an Alabama homicide also are interested in the gun. The caliber of the seized handgun was not disclosed by authorities.

In addition to six counts of first-degree murder filed in Maryland on Friday, Muhammad and Malvo also face capital murder charges in Montgomery, Ala. Police said a handgun was used to kill one woman and wound another there during a Sept. 21 robbery attempt outside a state-owned liquor store.

The handgun used in those shootings was not recovered, police said.

Montgomery Police Chief John H. Wilson said yesterday that police officers who responded to the shooting have identified Muhammad as a man who was standing over the two women, rummaging through their purses.

Wilson said his investigators now suspect that a third person may have been involved in the liquor store shooting.

Wilson said one officer started chasing Muhammad on foot. Meanwhile, a man at a fast food restaurant saw another person running in the same general direction and started chasing him but gave up a short time later.

During the police foot chase, a blue car emerging from the restaurant obstructed the officer, allowing Muhammad to escape.

"It looked like somebody buying a burger at the restaurant who was at the wrong place at the wrong time," Wilson said.

But he said the arrest of Muhammad and Malvo in a blue car gave rise to suspicions that it may have been a getaway vehicle in the Alabama shootings.

Wilson said he did not know whether Osbourne had any connection to his case.

"We're more than interested to see if he had any involvement in our case," he said. "He gives the blue car a whole new life for us."

The fact that Muhammad and Malvo were sleeping in their car when police apprehended them Thursday -- and that Baltimore police encountered Muhammad sleeping alone in the same car more than two weeks earlier -- has given rise to questions about how the two men supported themselves while in the Washington area.

Yesterday, D.C. Police Chief Charles H. Ramsey said police were reviewing a number of bank robberies, including one in which the description of a robber resembled that of one of the men.

"We're looking at some robberies," Ramsey said. "They had to eat somehow."

Investigators yesterday also sought to determine how Muhammad came to possess the Bushmaster .223-caliber rifle, which police say was recovered from his car and which has been linked by ballistics to 11 of the 13 shootings.

Muhammad was legally barred from owning a firearm as part of a restraining order his ex-wife obtained against him two years ago.

The owner of a Tacoma gun shop, Bull's Eye Shooter Supply, said his records show that the gun was in his inventory, but he said he had no paperwork indicating that it had been sold.

Although gun dealers are required by law to keep thorough records of all weapons transactions, the federal Bureau of Alcohol, Tobacco and Firearms has found discrepancies in the logs kept at Bull's Eye, the Seattle Times reported yesterday.

Bull's Eye owner Brian Borgelt said yesterday that he was trying to determine how the rifle allegedly used by the snipers left his store.

"That's what we're trying to find out," Borgelt said.

The question of who pulled the trigger in the 13 shootings also arose yesterday as a key issue in any prosecution.

Although Malvo is not eligible for the death penalty in Maryland because of his age, Muhammad would be if it was established that he fired the gun.

"It could be tough to get a jury to vote for the death penalty if they're not convinced who pulled the trigger," said Jim Shalleck, a Montgomery defense lawyer and former prosecutor.

Meanwhile, law enforcement sources said DNA evidence recovered from two of the crime scenes was found on a plastic bag and grape stems. They declined to reveal the sites where the material was recovered. The DNA has not been matched to that of the suspects, police said.

Authorities confirmed that Malvo tried to escape while being questioned in Montgomery County on Thursday. Left alone, handcuffed to the leg of a table in an interrogation room, Malvo broke the table and tried to climb through the ceiling.

"He was halfway out when an ATF agent pulled him back in," a source said. "Had he escaped, the only place he could have gone is into the next room over."

In Chicago, Nation of Islam leader Louis Farrakhan said at a news conference that Muhammad has been officially a member of the group since 1997 but has not been in contact with it since 1999.

"Certainly if he's found guilty of something like this, he would not be considered a member," Farrakhan said, according to the Associated Press.

A day after Montgomery County authorities charged Muhammad and Malvo with six counts of murder each, Spotsylvania County's chief prosecutor, William Neely, said that tomorrow he will begin the process of charging the two in the Oct. 11 slaying of Kenneth Bridges at a gas station there. Neely said he will ask juvenile authorities to charge Malvo.

Staff writers Michelle Boorstein, Hamil R. Harris, Sari Horwitz, Allan Lengel, Evelyn Nieves and Manuel Roig-Franzia, research editor Margot Williams and news researcher Lucy Shackelford contributed to this report.

Denise Johnson, widow of bus driver Conrad E. Johnson, attends his funeral at Glendale Baptist Church in Landover. She is supported by her sister Trina Young, left, and brother Kenny Young Jr. at the funeral, the 10th for victims of the sniper. Story on Page C1.