On the night of Nicola Cross' death, police detained then released Marcin Porczynski after reports a man had tried to enter her house.
Paranoid schizophrenic Porczynski later returned to the Hemel Hempstead home of Mrs Cross, 37, and stabbed her in 2015.
The Independent Police Complaints Commission found officers "followed applicable policies and guidelines".
Mrs Cross was stabbed on 14 September 2015 after Porczynski believed he had to kill her or his family in Poland would be killed.
Porczynski admitted manslaughter on the grounds of diminished responsibility in December 2016 and was sentenced to an indefinite spell at a psychiatric unit.
The IPCC investigation found Hertfordshire police officers were called to the area on the night of the killing following a 999 call from Mrs Cross and other residents reporting a man that was acting erratically and had tried to force entry to her home.
Porczynski was detained and then released by officers as there were no grounds to detain him further, the IPCC said.
He later returned to Mrs Cross' property and killed her.
The police watchdog concluded "the actions of no officer or staff member with Hertfordshire Constabulary could be considered misconduct".
The IPCC upheld a separate complaint, brought by Mrs Cross' husband Danny, that officers did not inform his wife that Porczynski had been released after he was first detained.
But the investigator added "this could not be judged by a tribunal to be misconduct".
IPCC operations manager Neil Orbell said: "My deepest sympathy goes out to Nicola's husband and children, and to all those involved in this awful event."
Danny Cross paid tribute to his wife (pictured together), who he spent 11 years with, and said they had an almost 'sickening perfect' life that was now destroyed
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