The Independent Office for Police Conduct (IOPC) has found Northumbria police officers did not properly investigate Alice Ruggles' complaints of stalking.
Alice Ruggles was murdered by her ex-boyfriend, Trimaan Dhillon, in Gateshead in 2016. She had complained to the police she was being stalked by Dhillon.
The IOPC found two officers had cases to answer for misconduct. It also found another officer did not properly investigate Ms Ruggles' final complaint.
Northumbria Police said the officers have now had additional training.
Watch Kris Jepson's report here:
The family of Alice Ruggles have previously called for more training to be given to officers in dealing with stalking.
Dhillon had stalked Ms Ruggles for months, despite being warned to stop by the police.
He was sentenced to 22 years in prison for Alice's murder.