Arthur Baldwin Jr., an officer with the U.S. Secret Service, parked his silver Nissan Altima on a road in Southwest Washington. He was not at work that December afternoon. He was waiting for a friend.

D.C. police said two people — one of them a teenager — spotted Baldwin sitting in his car in the 4700 block of First Street SW, on the edge of the Bellevue neighborhood. In an affidavit filed in court in a co-defendant’s case, police said the men mistakenly thought Baldwin was there to buy a large amount of drugs.

The teenager, Maurice Bellamy, announced that he wanted to “pull a move” — a street term for a robbery, witnesses recounted for police, according to the court document. It was just before 3 p.m. on Dec. 15.

Police said Bellamy, 17, and Charles Sims, 29, robbed and then fatally shot Baldwin. Afterward, they buried .22- and .38-caliber pistols in woods and near a creek off First Street, marking the spot with a blue ribbon tied around a tree, the court paper states.

They made off with an iPad, but no cash, according to the document.

At a Wednesday news conference, D.C. Police Chief Cathy L. Lanier announced that Bellamy has been charged with first-degree murder in Baldwin’s death. The teen now faces charges in two unrelated slayings.

Bellamy was in jail awaiting trial on charges that he fatally shot 15-year-old Davonte Washington on the platform of the Deanwood Metro station March 26 over what police said was a glance perceived as disrespectful. Washington was on his way to get an Easter haircut with his mother and two younger sisters.

Lanier said the charges in Baldwin’s slaying were the result of months of work by detectives.

“These types of investigations take a lot of time,” she said. “There has been an awful lot of investigative work that has been done on this case in the past couple of months.”

Ming Cardwell, one of Baldwin’s three sisters, said it was difficult to discover that the alleged shooters may have mistaken her brother for a drug dealer and that the shooting “was a case of mistaken identity, that he was in the wrong place at the wrong time. That just makes it worse.”

Cardwell, who lives in suburban Cincinnati and mentors troubled youths, said her brother had started a construction company after he was suspended from the Secret Service pending an investigation over a domestic incident with an ex-girlfriend. She said she doesn’t think he “knew how dangerous the area was that he was in” on the day he was shot.

Bellamy, Sims and another man walked up to Baldwin’s car, according to the police affidavit. Bellamy was the first to approach, knocking on the window and asking Baldwin the time, according to one witness’s account included in the records. Bellamy then pointed a pistol at Baldwin.

One member of the trio opened the passenger door, the other the driver’s door, the third the trunk. A witness told police that the men rummaged through the car for about three minutes when Baldwin suddenly got out.

Bellamy shot twice, the police affidavit says.

[U.S. Secret Service officer killed during robbery in Southwest]Police said Sims then fired. Baldwin, struck in the torso by five bullets, ran across the street to a grassy area near the woods and collapsed. The gunmen ran.

One witness told police, according to the affidavit, that he later heard Bellamy tell Sims that “he shot the decedent because he tried to ‘buck’ on him and take his pistol.” The same witness said Sims told him “he fired because Bellamy was struggling with the decedent and once Bellamy shot [Baldwin], Sims couldn’t leave him there alive.”

Sims, of Southeast Washington, was charged Tuesday with first-degree murder.

Baldwin’s slaying left his family angry and in mourning. Because her brother had been suspended from his job with the Secret Service’s Foreign Mission Branch, Cardwell said his funeral lacked the pageantry befitting the killing of a law enforcement officer. There was no 21-gun salute, no American flag draped across the coffin.

Although Cardwell said the Secret Service privately offered the family support, the agency kept its public comments to a two-sentence statement: “We are aware he was killed. We express our condolences to his family.”

“Because he was suspended, they were hesitant to acknowledge him,” Cardwell said. “This was an officer who was committed to his work. He worked in the White House and took an oath to protect the president, only to be gunned down in the middle of the street. He never got the attention that was warranted.”

The police affidavit makes clear that Baldwin, 30, of Upper Marlboro, did not have drugs and was not in the area buying drugs. Authorities confirm that he was simply waiting to meet a friend when he was ambushed. Officers found him at 2:51 p.m., lying in the grass, struggling to breathe. He died at a hospital at 3:23 p.m.

His vehicle was found with the front door and trunk open.

A silver watch was found near where Baldwin fell. His iPad and wallet had been taken. Court papers say a witness told police that Bellamy found no cash in the billfold, so he tossed it down a storm drain.