A man serving life for smothering his 5-year-old son in Keelung has kept sending threatening letters to his ex-wife's family from behind bars, and the detention center admits he slipped past its screening.

• He was given life without parole last year for killing his young son with a pillow.
• His ex-wife's family got a letter in June threatening to wipe out the whole family, then another in August after he was moved to Taipei Detention Center.
• Staff say he faked the recipient details to look like he was writing to a friend, so the mail passed review.
• The center only received the family's emergency protection order after that letter had already gone out.

Outlook: Prosecutors will look at criminal intimidation charges, and the center says all his mail is now blocked and high-risk inmates face tighter letter checks.