Thetis Nixon-Jackson, wife of Alonzo Jackson, is embraced her husband’s cousin, Linda Jenkins, during a vigil Sunday. (Amanda Voisard/For the Washington Post)

A man has been arrested in the killing of Alonzo Jackson, 68, the retired Safeway employee and former Marine who was shot to death last week in Prince George’s County after pumping gas, police said Sunday.

Jackson was shot Wednesday at a gas station in the 4900 block of Marlboro Pike in Capitol Heights. His car was taken, and the vehicle later was found abandoned. It had been set on fire.

County police said Demarko Wheeler, 20, of the 2000 block of Stanton Road in Southeast Washington, was arrested Sunday in the District. He was charged with first- and second-degree murder and related charges, police said.

They said Wheeler was in D.C. police custody and was awaiting extradition to the county. The 7 a.m. shooting occurred a little more than a mile from Jackson’s home in Southeast Washington.

After dropping off his wife at work, Jackson was at the gas station preparing his black 2015 Dodge Charger so he could take his best friend to a medical appointment. The friend was having complications from knee replacement surgery, Jackson’s son said.

Demarko Wheeler. (Prince George’s Police Department)

A vigil for Jackson was held at the shooting site Sunday evening, and when the news of the arrest was announced, the crowd cheered. Balloons were released. At the vigil, family members wore shirts emblazoned with Jackson’s face that read “gone but never forgotten,” and “we miss you.”

Police said a preliminary investigation showed that Jackson had just filled up the car when he was approached and shot. The killer then drove off in Jackson’s car, police said. It was not clear what, if anything, provoked the gunfire.

Although Jackson prized the vehicle, his son, Alonzo Jr., 47, recalled Sunday that he played down the importance of such possessions.

“He had to preach it to me,” the son told a reporter: “If somebody wants that car, if somebody’s gonna rob me for that car, he always said, ‘Give it to him. Just let him have it.’ ”

A tip helped identify a suspect in the case, police said Sunday.

Police had posted a brief video clip online Friday of a man they described as a suspect in the killing.They also said that a $25,000 reward was being offered.

A nephew holds a family photo showing Alonzo Jackson and Jackson's wife. (LaVendrick Smith/The Washington Post)

Jackson was a father and grandfather. Raised in the District, he was a longtime Redskins fan.

“He was the sweetest guy in the world. Everybody looked up to him,” said Rodney Nixon, 39, Jackson’s nephew and godson. “Just a sweet old man. For this guy to kill him for nothing was just senseless. Didn’t even want the car. Everybody loved him.”

Jackson served as a sergeant in the Marines for three years beginning in the late 1960s, his brother said, adding that he had been stationed in places including Norfolk and Nova Scotia, Canada.

The Rev. Reginald Jackson, 64, of Southeast remembered him as a “plain and simple” guy who loved the District and was “ just a D.C. boy.”

He liked the weather, “even on a a day like today,” Jackson’s brother said.

It was hot and humid during the vigil, with temperatures hovering around 90 degrees. Still, the crowd appeared to number in the dozens.

The slain man’s nephew and godson Clifton E. Powell Jr. denounced the killing to The Washington Post last week as a “senseless act of ­violence.” He said it had outraged relatives and friends.

“It was not his time to die,” Powell wrote of his uncle, “especially [for] a materialistic item such as his car!”

Speaking for the family, Powell wrote: “We demand justice.”

Family members and friends had altered their Facebook profile photos to show an image of the slain man.

Beneath it, in red lettering, was the word “justice.”

In a statement read for her at the Sunday evening vigil by a relative, Jackson’s wife of 10 years, Thetis Nixon-Jackson, addressed his killer.

She said a brief conversation with Jackson would have shown that “he was not the type of person you would have killed.”

The statement said: “You have no idea the pain you have caused me, my family, friends, and this community. We expect justice to be served. And maybe one day we will forgive you. May God have mercy on your soul.”