A man who shot and killed a witness set to testify at a 2002 Brooklyn murder trial was sentenced yesterday to 25 years to life in prison.

Judge L. Priscilla Hall gave Trevis Ragsdale the maximum possible sentence after listening to legal arguments and an impassioned plea for severity by the slain witness’s mother.

Ragsdale, 20, was convicted in June of second-degree murder for gunning down Bobby Gibson by a stoop in Bedford-Stuyvesant.

Mr. Gibson was two days away from testifying against a man accused of a 2001 schoolyard slaying.

Mr. Gibson’s murder shook the Brooklyn courthouse and raised broader questions about the ability of prosecutors and police to protect witnesses who step forward and testify in murder cases.

Yesterday, his mother, Sylvia Gibson, told the judge in state Supreme Court in Brooklyn that it was “unbearable” that her son was “shot in the back because he tried to do what was right.”

Ms. Gibson asked Judge Hall to jail Ragsdale for “as long as possible…. There are no words to discuss the pain and devastation this family has gone through. I lost my only child.”

Ms. Gibson is suing the Brooklyn district attorney’s office for failing to do enough to protect him.

As she spoke, Ragsdale, wearing jeans and a T-shirt, leaned on his hand, avoiding eye contact with Ms. Gibson. Through his lawyer, Ragsdale asked for leniency from Judge Hall, who could have sentenced him to 15 years in prison.

Before making her decision, Judge Hall asked Ragsdale if he had anything to say.

“No,” he mumbled, shaking his head.

“Are you sure?” the judge asked again.

“Yeah.”

Ragsdale’s defense lawyer, Paul Lieberman, told the judge that his client had psychological and emotional problems but still could be rehabilitated.

He denied that his client is a member of the Bloods gang and presented the court with a letter signed by more than 200 people who said Ragsdale is a basically good person.

“We all have many sides to our personalities,” Mr. Lieberman argued.

Assistant District Attorney Kyle Reeves called Ragsdale a “cold-blooded murderer” and “a man of violence” who had been caught repeatedly with guns.

“What we are left with is a violent, dangerous menace,” Mr. Reeves said. He urged the judge to give Ragsdale the maximum sentence in order to deter similar crimes and “protect the people of Brooklyn.”

Judge Hall said she had considered how Ragsdale behaved in court in determining his sentence. During the trial, Ragsdale once cursed at her.

After court, Ms. Gibson said she was satisfied with the sentence.

“I can finally put this behind me,” she said.

Ragsdale’s mother, Rosa Ragsdale, said that she hopes her son will win on appeal.

“God has last say on everything,” she said.

The sentencing still left a key issue in the case unresolved. Prosecutors had accused Ragsdale of shooting Mr. Gibson to silence him and scare other witnesses. But jurors rejected a first-degree murder count that accused Ragsdale of killing Mr. Gibson because he was a witness.

According to prosecutors, Ragsdale wanted to help another man, Dupree Harris, who is the half-brother of the man eventually convicted of the 2001 schoolyard slaying. Prosecutors have never offered evidence of a direct link between the two men.

Mr. Harris was convicted last month of bribing three witnesses to stop them from implicating his half-brother, Wesley Sykes.