Justin Martin, pictured, was shot dead by Sarah McKinley as he tried to break in to her house

 The 18-year-old Oklahoma widow who shot dead a man after he broke in to her house won't face charges over his death, police said yesterday.

The man was looking for the prescription drugs belonging to Sarah McKinley's dead husband, an affidavit released on Wednesday revealed.

Ms McKinley's 58-year-old husband Kenneth had died from lung cancer on Christmas Day, leaving behind their three-year-old son Justin, and an arsenal of cancer medications.

Justin Martin, 24, was addicted to prescription drugs, his friend told police, and was looking for his next fix.

Painkillers had gone missing from their home in recent weeks, Ms McKinley said, and suspected someone was breaking in.

The young widow told NewsOK.com that it is not customary to lock one’s doors in the rural community of Blanchard, Oklahoma.

Ms McKinley said Martin had knocked on her door the night of her husband’s funeral when it was ‘pitch black out.’

She didn’t recognise him but said he was acting strangely. Once he realised she had company, he left.

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Make my day: Sarah McKinley shot and killed an intruder on New Year's Eve to save her three-month-old son, as reenacted for Good Morning America in January

Martin returned New Year’s Day with a 12-inch hunting knife. Ms McKinley told NewsOK.com that it took Martin 21 minutes to break into her house.