The Austin police department is firing an officer who shot and killed a naked and unarmed teenager in the street last month.

Geoffrey Freeman, a 10-year veteran of the force, was responding to calls about disturbances in a neighbourhood in the north of the Texas city on 8 February and came across David Joseph, a 17-year-old high school senior.

A police department spokesman said that Freeman shot Joseph within seconds of encountering him at about 10.30am. The officer said the teenager charged at him and he feared for his safety, but activists and Joseph’s family questioned why Freeman did not use his Taser or call for back-up and the help of officers specially trained to deal with people in apparent mental health crises.

According to police, dashcam video does not show the part of the encounter when shots were fired. But at a press conference on Monday, Austin’s police chief, Art Acevedo, told reporters that a disciplinary panel concluded after a hearing that Freeman violated departmental policies on response to resistance, general conduct and handling “substance-induced excited delirium”.

An autopsy report found that Joseph had both the anxiety-treating drug Xanax and marijuana in his system and that he was apparently shot three times from a distance of no more than two to three feet.

“David would be alive with his family today if Freeman wasn’t the only officer on the scene, and if he’d been trained to use non-lethal force first,” the family’s attorney, Jeff Edwards, told KXAN local news.

Acevedo said the panel “did not feel that the officer’s use of deadly force under the circumstances met the ‘objectively reasonable’ standard”. He said that Freeman will appeal but he is confident that the department’s decision will be upheld and the firing will be formally confirmed. “Right now he’s indefinitely suspended without pay pending a hearing and pending a final ruling from the arbitrator,” he said.

Freeman, 42, had previously received several commendations from senior officers for his work. “I urge people to not demonize a person, Officer Freeman is a person that but for this horrible set of circumstances served honourably, but based on these set of circumstances we do not believe he should remain on the department,” Acevedo said.

Both the officer and the teenager are black. The death sparked a series of community protests and demands for a swift investigation. A state police union, the Combined Law Enforcement Associations of Texas (Cleat), criticised Acevedo for holding a press conference in the wake of the death that featured Black Lives Matter activists, among others, and said the firing was “unjust and politically motivated”.

“Rather than taking the time needed to conduct a thorough and impartial investigation into the tragic events of Feb 8, 2016, Chief Acevedo instead chose to send signals to the news media, political activists, rank-and-file police officers and even a cadet class that he intended to fire Officer Freeman no matter what the facts in the case might show,” Cleat’s president, Todd Harrison, said in a statement.

Acevedo said the full details of the internal affairs inquiry would not be released publicly because of an ongoing investigation by the Travis County district attorney’s office, which is expected to result in the case being put before a grand jury to decide whether Freeman should be indicted on criminal charges.

Last week a Dallas-area police officer was arrested on charges of murder and aggravated assault, accused of fatally shooting a 16-year-old and wounding another juvenile while off duty following a car chase.