Meyder Bladimir Yuman told his mother he was going for a walk in his Lewisdale neighborhood before setting out with a friend Thursday evening, relatives said. He was less than a mile from home when someone pulled up in a car, got out and shot the 18-year-old to death, authorities said.

“I don’t have any idea” what happened, Norma Yuman, Meyder Yuman’s mother, said in Spanish through an interpreter. “He was not a troubled boy.”

Prince George’s County police said they have no suspects or definitive motive in the slaying, the 19th in Prince George’s this year, compared to 10 at the same time last year. Even more troubling, the victim was a teenager, the fifth killed this year.

Lt. William Alexander, a Prince George’s police spokesman, said that investigators think Meyder Yuman was walking in the 7200 block of 24th Avenue about 7 p.m. when someone pulled in up in a tan or gold four-door Toyota Corolla with Maryland tags. The person — or people — in the car got out, confronted Yuman, then shot him more than once, Alexander said.

Yuman’s slaying is the latest in a series of unrelated killings of young people that has jolted Prince George’s. In the same general area in December, 14-year-old Eliezer Reyes, a student at an alternative school in Largo, was fatally shot as he walked with members of the so-called Lewisdale Crew. Police have arrested the suspected attackers in that case and said they believe they were members of the rival Mara Salvatrucha, or MS-13 gang. Alexander said detectives were exploring the “possibility” that Yuman’s slaying was gang-related, although they had not reached any conclusions.

Six Prince George’s County students have been killed during the 2012-13 school year as a result of violence.

Norma Yuman, 37, said that she knew gangs were in the area, but that she had asked her son repeatedly whether he had joined one and he insisted he had not. Maria Yuman, Meyder Yuman’s aunt, said he was not the type to participate in that activity.

“We know him, for sure,” Maria Yuman, 37, said. “We think he wouldn’t do that.”

Norma Yuman said her son, who came to the United States from Guatemala when he was 3 years old, had attended Northwestern High School, although he left in 2009 to work construction jobs. His aunt said Yuman intended to get his GED soon.

John Arhar, a neighbor, said he worked with Yuman and his stepfather on roofing and other jobs and the teen had become a friend. He said Yuman was “outgoing, ambitious” and “very smart,” and the two would talk frequently about philosophical questions.

“I saw a lot of him in me,” Arhar said. “He was curious about, how can I say, how life is going to unfold.”

Norma Yuman said her son was with one friend when he was shot, and she was reaching out to others to see if they knew of any disputes he had with anyone. So far, she said, she had turned up nothing.

“All of them say he really was a good kid,” Norma Yuman said. “Very polite with everybody.”

Magda Jean-Louis and Ovetta Wiggins contributed to this report.