He received a threatening letter.

他收到一封恐吓信。

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Dorner claimed Williams was harassing him and sent a threatening letter to his home.

NPR: Court File: LA Ex-Cop Was Disturbed, Self-Obsessed

Another variety of threatening letter was sent to taxpayers under audit.

FORBES: You have the right to be terrified

In 1967, the year Lillehei left Minnesota to become chief of surgery at New York Hospital-Cornell Medical Center, the IRSsent a threatening letter.

FORBES: The IRS Gets a Doctor