(CNN) Fighting through tears, Julianna Saldivar is asking why.

Her boyfriend, 16-year-old Jose Raul Cruz, was shot and killed by an off-duty Farmers Branch, Texas, police officer on Sunday in Addison.

"Why did he have to shoot him?" she asked at a news conference, according to CNN affiliate KDAF . "Why did he have to take his life away?"

Authorities, asking the same question, have arrested the officer, Ken Johnson, in connection with the shootings of Cruz and another teen who survived.

Attorney: Officer feared for his life

Johnson faces one count of murder and one count of aggravated assault, the town of Addison said in a statement.

He was being booked into the Dallas County Jail. His bond has been set at $150,000 according to CNN affiliate KTVT.

Mug shot released of Farmers Branch officer charged with murder, Ken Johnson. Bond set at $150k. pic.twitter.com/ppVnL0AKmB — CBSDFW (@CBSDFW) March 17, 2016

"Based on our investigation thus far we had probable cause to make the arrest this evening," said Paul Spencer, Addison's police chief.

"But this is a rapidly-evolving situation and it remains an active investigation. Therefore, we are refraining at this time from discussing the evidence collected thus far or about the specifics of this case. We expect there are still several weeks of additional investigative work to do," he said.

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 also 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 also 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.

'I want justice'

Cruz's family met with police on Tuesday, according to KDAF. Relatives and friends gathered Wednesday to mourn at a service at a Houston funeral home.

"I want justice," Cruz's mother told reporters on Wednesday, according to a translation from family spokesman Carlos Quintanilla.

Quintanilla said there were over 25 witnesses to the incident 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.

"He wasn't just our friend -- he was our brother," one of his friends said. "And they killed our brother for no reason, just cause he had a fast car, just cause he had a nice car."

"He worked for his money."