Prince George’s County police said Tuesday they believe robbery was the motive in the killing of a 15-year-old Suitland High School student.

The student, Charles Michael Walker Jr., was found slain Monday afternoon on 28th Avenue. He had been shot once, according to Prince George’s County Police Chief Mark A. Magaw, who held a press conference Tuesday afternoon.

Walker was found about 4:20 p.m. in the 4000 block of 28th Avenue, a heavily used residential street near the Marlow Heights and Iverson Mall shopping centers. Magaw said at Tuesday’s press conference that Walker had been carrying a bag containing shoes; however, the shoes were not taken, Magaw said.

Magaw said there was no indication that Walker knew the assailant.

Police said they were sent to the scene after the youth had apparently collapsed and was seen lying on a sidewalk or lawn. A relative said he lived near the site.

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

Neighbors on the street of red brick duplexes said they had heard nothing. At least one longtime resident called the street generally problem-free.

The county schools expressed sadness in a statement, calling senseless violence against children “all too common.”

Dan Morse contributed to this report.