Michael Lane is accused of slitting Miss Grice's throat
Officers warned tyre fitter Michael Lane, 27, to stop harassing Shana Grice, 19, and gave him an official caution but it failed to prevent her death, a jury was told.
The pair had a brief relationship but after they split up Miss Grice became increasingly alarmed by his obsessive behaviour, it was claimed.
Philip Bennetts, QC, prosecuting at Lewes Crown Court, said the pair met in 2015 when Shana began working at Brighton Fire Alarms, where Lane, from Portslade, Brighton, also worked.
But the relationship had become on-off and she ended it. She later told police he had stolen a key letting himself into her home in Brighton.
Miss Grice was terrified of Lane's obsessive behaviour towards her
He stood over her as she hid under her duvet and placed a tracker on her car to monitor her movements, she said.
Officers contacted Lane and issued the caution. During a later meeting, Miss Grice told Lane that she wanted a relationship with another ex-boyfriend, Ashley Cooke.
His car was later found vandalised with a note reading: “Dear Ash, Shana has and always will cheat on you. “Happy New Year.”
Mr Bennetts added: “Unfortunately Michael Lane had become obsessed with Shana.
“That obsession with her translated into killing her. He would not allow anyone to be with her.”
Michael Lane had become obsessed with Shana
Philip Bennett, prosecuting
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Miss Grice’s car tyres were slashed and Lane pulled her by the hair in the street, the court heard.
It was after he collected his belongings from her home last summer he stole a door key, Mr Bennetts said.
“About 6am he went to the house and let himself in. She was in bed when she heard noises and footsteps approaching her bedroom,” Mr Bennetts said.
“She hid under the duvet and heard her bedroom door open and a male breathing in her room. He remained for about a minute before he left.”
Ashley Cooke found a note attached to his vandalised car
When he left she looked out the window and saw Lane walking off, the court heard.
He was arrested and cautioned but Miss Grice then received heavy-breathing calls from an unknown number. Lane also told a friend at the time “she would pay” for what she had done.
He had arranged for alerts to be sent to his phone from the tracking device on her car to trace her movements.
Lane allegedly tried to burn down Miss Grice's room
After she finally told him the relationship was over, Lane texted saying: “It has to be goodbye forever.” Mr Bennetts said on August 25 Lane watched Mr Cooke leave Miss Grice’s home and added: “Michael Lane went in.
"He slit her throat and no doubt intending to destroy any evidence of his presence in the house, he set fire to the room in which she was in.
"In fact, the fire did not destroy the room.”
After she failed to appear at work a friend called round, only to spot a bloody footprint on the door of the property.
When friends entered the house it was full of smoke. Mr Bennetts said: “Through the hazy smoke they were able to see a body lying face down on the bed.”
A post mortem examination revealed the cause of death was a 4in slash to the throat.
Mr Bennetts said: “Shana had been dead before the fire broke out.” Lane denies murder. The trial continues.