It was something any 3-year-old boy might do: Run out to the car, climb inside and greet his grandmother.

“He was the sweetest little boy,” one neighbor said Thursday. “Every time you [saw] him, you just wanted to give him hugs.”

In what Prince George’s County police are describing as a tragic accident, Dominic Watkins somehow fell out of the car as it drifted backward, and he was run over and killed.

Things unfolded in a matter of seconds. Police said Dominic’s father had pulled up in the car, left it running and got out to pick up Dominic and his 2-year-old sister. After Dominic had run ahead, the father saw the car rolling, hurriedly put down his daughter and jumped inside to try to stop it.

“He was right there, and that’s going to hurt him the rest of his life,” said Webster Moat, another neighbor.

The accident occurred about 7:30 p.m. Wednesday outside an apartment building along Audrey Lane in Oxon Hill. Two neighbors said the family was set to celebrate Dominic’s fourth birthday on Thursday. Police are continuing to investigate how Dominic died but do not suspect that alcohol or foul play was a factor. Officials did not release the father’s name.

By Thursday morning, neighbors had erected a memorial on the patch of asphalt where the Dominic died, decorating it with more than a dozen stuffed animals, flowers and candles. There were also balloons. Two said “Happy Birthday.”

Marcia Reid, 28, another neighbor, said she was home Wednesday evening when her boyfriend told her that he heard a loud noise, as if someone had been struck. Reid went outside and saw a woman holding a young boy.

Reid could see his head was bleeding and hear him gasping for air. He appeared to be losing consciousness. By the time paramedics arrived, Reid said, the boy was not breathing. They performed CPR.

Dominic was taken to a hospital, where he was pronounced dead.

“It was just so sad,” Reid said. “I can’t even get his face out of my head.”

Cpl. Maria McKinney, a police spokeswoman, said the boy’s grandmother had been inside the car. Police officials described his father as moving so quickly that when he put his daughter down to try to stop the car from rolling, she fell and suffered injuries that required hospitalization. The injuries were not considered life-threatening, police said.

The father, who was not injured, could not be reached to comment. One of his neighbors, Tay Feb, said Dominic’s father was at the hospital after the accident. “Can’t do nothing but cry today,” Feb said outside the apartment building.

Feb was wearing a jacket with a picture of Dominic affixed, one of 20 such garments he had made for family and friends. “RIP Pa,” the image said, a reference to Dominic’s nickname.

Kristal Hinton, who used to live in the apartment complex and is good friends with the boy’s family, said Dominic was like any other kid and used to play outside with her children. “He was the love of their life,” she said of Dominic’s family.

Eddy J. Palanzo contributed to this report.