Seated beside her husband in the back of a Dorchester courtroom, Laura Dos Santos wept as a prosecutor on Monday described how two teenagers “trapped” her 16-year-old son before shooting him to death last week.
At the front, on opposite sides of the district courtroom, Genneane Gennis wiped tears and relatives consoled Retha Moody as Judge Thomas Kaplanes ordered both of their sons — 16-year-old Du’Shawn S. Taylor-Gennis and 14-year-old Raeshawn X. Moody — held without bail following their arraignment on charges of murder and unlawful possession of a firearm.
Both boys, who remained in a room away from public view, pleaded not guilty to the slaying of Jonathan Dos Santos. The two are being tried as adults. Their mothers had turned them in to police on Saturday, decisions that city officials and community leaders praised as courageous.
The lives of the three women came together in a dramatic and bloody way shortly before 8 p.m. on Wednesday, at the intersection of Fuller and Washington streets.
In court, Suffolk Assistant District Attorney Jennifer Hickman said police had recovered a private surveillance video in which images of both suspects are clear.
She said the video also shows the two working in concert to “trap” the victim between them. It was not immediately clear what the alleged assailants did to trap Dos Santos or what the motive for the shooting was.
She said the duo appeared to be waiting for someone. They walked up and down the sidewalk together, laughed, got on their cellphones and even looked at the surveillance camera, Hickman told the court.
At about 7:53 p.m., the two teenagers separated. One youth walked across the street and remained visible while another hid on the other side of the street.
“Within minutes of setting up the trap, [Dos Santos] arrived,” Hickman said, as Laura Dos Santos began sobbing. “The video shows, as he rode up the street on his bike, he approached that corner and observed the defendant on the sidewalk.”
Dos Santos stopped on his bike to acknowledge one of the teens. That teen makes a gesture that startles Dos Santos, who is seen in the video ducking and beginning to flee, Hickman said.
The other boy left from “where he had been hiding and came up from behind and took out a firearm and shot five times,” Hickman told the court.
Prosecutors did not identify the suspects by name seen in the video.
Hickman said the Dorchester teen was fatally struck once in the back.
He died two doors down from his aunt’s home on Fuller Street and a couple of blocks away from the three-decker home he shared with his mother, father, and 9-year-old sister, Jennifer.
Friends and relatives wore buttons that bore a picture of Dos Santos and the words “Peace for Jo Jo,” Jonathan’s nickname.
“The family is not upset with the other families. They just want the young men held accountable for their actions,” said family friend Joao Depina, 36. “It’s sad. It’s a three-way loss.”
On Saturday morning, police stopped by Genneane Gennis’s home on Mora Street, a block away from where the two boys allegedly ambushed Dos Santos, and asked to speak with her son, attorney Michael Doolin told the court. She went and picked him up and brought him to the police station.
“She did that as a good citizen, as a good mother,” Doolin said in an effort to negotiate bail, adding that Taylor-Gennis was a student at English High School. “It shows that he is the type of person who comes from the type of family that is not going to run away from the trial in this case or any other court proceedings.”
Earlier Monday, Gennis broke down in tears at her house as two people rushed her into a waiting car. She talked briefly about her decision to turn her son in to authorities.
“It was very hard. Another boy is dead. I can’t . . .” she said.
A man who only identified himself as Coach G said he has known Taylor-Gennis since the boy was 9 years old.
“He’s a quiet kid,” the man said. “Nice.”
The man said he cried when he heard the crime that Taylor-Gennis was accused of committing — and even more so when he realized who the homicide victim was.
“I know they’re hurting. I know people in the family. It’s sad,’’ the man said of the Dos Santos family. But “he didn’t do this. That’s why we have the justice system.’’
Officers also turned to Retha Moody and asked her to bring her son into the police station so that they could talk to him — and they later informed her of the charges, attorney Michael C. Bourbeau told the Globe after Monday’s hearing.
“There was nothing to hide,” Bourbeau said. “It’s simply them being the citizens they are and doing something that’s appropriate.”
Retha Moody declined to comment. She and her son have been staying at the St. Ambrose Family Inn, a shelter in Dorchester. The family had been evicted from their apartment on Hiawatha Road in 2013 and was ordered last fall to pay nearly $2,600, court records show.
Moody herself was a victim of violence in 2003 when a gunman shot five people, leaving one dead in Dudley Square. She was hit in the leg.
Concerns have been raised about the ages of the boys and the nature of the crime they stand accused of.
Both will be kept in the custody of the Department of Youth Services while their cases play out in court, officials said.
Jan Ransom can be reached at jan.ransom@globe.com. John R. Ellement can be reached at john.ellement@globe.com. Astead W. Herndon can be reached atastead.herndon@globe.com.