First he heard a thud. Then screams. Then Jim Sugent saw a 4-year-old boy lying in the middle of Stevenson Avenue in Alexandria.

Police say the child, who was identified Friday as Jacob Choi, had run into oncoming traffic May 4 and was fatally struck by a 2010 Jeep Commander. The driver, who is 84, has not been identified by police. He has not been charged in the accident, which police said remains under investigation.

Jacob’s mother, who lives in Alexandria, ran into traffic after him and also sustained injuries, which were not serious, police said. Her name was not released because she is a witness in the investigation, police said.

In the minutes after Jacob was hit, Sugent, a 47-year-old former respiratory therapist trained in CPR, said he tried to save him.

Although the boy was unconscious, he still had a pulse, and Sugent continued to breathe into his mouth for more than two minutes until ambulances arrived. The boy had a serious head gash, and a driver grabbed a first-aid kit from his car and did his best to wrap the boy’s wound, Sugent said.

The Jeep’s driver had gotten out of his vehicle and appeared to be in shock, according to Sugent and another witness. “He was pretty distraught,” Sugent said, adding that the man wanted to help but didn’t know what to do. “He was running back and forth between the car and the boy.”

Emergency workers put the boy on a stretcher to take him to Inova Fairfax Hospital. After they left, Sugent went inside his nearby condominium, sat down and started to cry. He knew the child was badly injured.

“He was the cutest little boy,” he said.

He took a shower to wash off the blood. It wasn’t until later that day that he went back outside and learned what had happened from a crowd of reporters and police officers.

The little boy had died.