(CNN) An off-duty police officer charged with murder in a 16-year-old boy's fatal shooting resigned Thursday, said the police chief in Farmers Branch, Texas.
Officer Ken Johnson faxed his resignation to Chief Sid Fuller, according to a copy of the letter provided by the department.
"It has truly been a pleasure to work for such a fine department. The staff, officers, and administrators of the department are among the finest. I have taken great pleasure in working alongside these individuals," Johnson wrote.
The police chief said an internal affairs unit continues to investigate the March 13 shooting in which teenager Jose Raul Cruz was shot and killed by Johnson while the officer was off-duty.
Another teen survived the shooting, authorities said.
"Notwithstanding Officer Johnson's resignation we will continue our thorough review of the information collected and work to conclude the investigation; however, no official employment action will be taken in light of today's resignation," Fuller said in a statement.
The shooting occurred in Addison, where its police continue a criminal investigation into the shooting, Fuller said.
Cruz's family couldn't be immediately reached for comment.
The family has expressed anger after Johnson was freed on $150,000 bond, and the family accuses Johnson of shooting at innocent teenagers, according to CNN affiliate KTVT.
Johnson faces one count of murder and one count of aggravated assault, the town of Addison said in a statement.
After the shooting, Cruz's girlfriend wept and asked why.
"Why did he have to shoot him?" Julianna Saldivar said at a news conference, according to CNN affiliate KDAF. "Why did he have to take his life away?"
According to CNN affiliate KTVT, Johnson allegedly saw suspects burglarizing a vehicle and chased them in his own vehicle after they took off.
The 16-year-old driver was shot and killed, KTVT said. His passenger was reportedly injured.
Christopher Livingston, Johnson's attorney, told KDAF that the officer "was in fear of his life when he took the shots and he believed that that fear was justified."
In a statement to CNN, Livingston said he's disappointed the case was not taken to a grand jury.
"For decades these cases have been presented to a grand jury in Dallas County. Officer Johnson has fully cooperated with the investigation," Livingston said. "He looks forward to presenting this case to 12 unbiased jurors.
Cruz family spokesman Carlos Quintanilla has said there were more than 25 witnesses and video surveillance.
"There was no 911 call. There were no officers in pursuit," he said. "The evidence is so overwhelming that there's nothing that could be done except an indictment or an immediate arrest."
The car Cruz was driving belonged to him -- it wasn't stolen -- Quintanilla said.
Last week, Cruz's relatives and friends gathered to mourn at a Houston funeral home.
"I want justice," Cruz's mother told reporters on Wednesday, according to a translation from Quintanilla.