A former police officer did not adequately investigate reports a woman was being stalked by an ex-boyfriend who went on to murder her.
Shana Grice, 19, complained five times to Sussex Police about Michael Lane before he slit her throat in 2016.
The failings of PC Mills, whose full identity can not be reported, amounted to gross misconduct, a force disciplinary panel ruled.
PC Mills, who resigned last week, had denied the accusations.
Had he not resigned, the panel said he would have been dismissed from the force.
Ms Grice, who was later fined for wasting police time, reported on 9 July 2016 that Lane had stolen her backdoor key, crept into her bedroom and watched her sleeping, the panel heard.
He was arrested but, despite there being a history of escalating reports of stalking and harassment, PC Mills, the investigating officer, did not review case notes before questioning him.
Lane was cautioned and warned to stay away from Ms Grice.
PC Mills also failed to respond to reports made by Ms Grice on July 12, when she said Lane had been following her in his car, the panel was told.
She was not called back and a few days later received a letter stating the "case was closed".
It was the last time she contacted police before she was murdered by Lane at her Brighton flat in August.
Lane, who was jailed for a minimum of 25 years in 2017, had placed a tracker device on her car during a campaign of harassment.
The force's lawyer Louise Ravenscroft said Ms Grice had reported she was "scared even to leave my house".
According to friends, Ms Grice had been angry and "could not believe they [the force] had dropped it", Ms Ravenscroft said.
"As a result, she never reported continuing complaints of stalking."
During an interview with the Independent Office for Police Conduct (IOPC), PC Mills said he did not know why he failed to call her back.
When questioned, Mills admitted he had been "alarmed" by some of Lane's behaviour and said his failure to properly question him had been an oversight.
Panel chairman Chiew Yin Jones said PC Mills' failings may have "ultimately contributed in the circumstances which contributed to the tragic death of Ms Grice".
"In his dealings with Ms Grice, the officer failed to recognise her vulnerability," she said.
The family's lawyer Andy Petherbridge, of Hudgell Solicitors, said it was the "right decision" but was "too little, too late" for Ms Grice.
The panel ruled PC Mills' full name should be withheld to protect the "feelings and the welfare" of his young family.
PC Trevor Godfrey, who retired from duty in December 2017, was due to face allegations of discreditable conduct on Tuesday, but the hearing was postponed until further notice.
An internal misconduct hearing for a serving, unnamed officer, will be held on 17 May.