Threatening his girlfriend with a buck knife last September has cost a Knox man 5 1/2 years in prison.

Edward William Withee III, 22, formerly of Augusta and Oakland, pleaded guilty Wednesday in Kennebec County Superior Court in Augusta to criminal threatening with a dangerous weapon; assault; tampering with a witness, informant, juror or victim; and violating conditions of release.

When the threatening incident occurred on Sept. 24, 2007, in Oakland, Withee was on probation for previous convictions for robbery, burglary and criminal mischief involving a home invasion.

In a police affidavit, the victim of the threatening told investigators, “He tried to kill me last night.”

Withee was sentenced Wednesday to 5 1/2 years in prison followed by four years’ probation.

He also was fined $300.

Withee also had been charged with gross sexual assault, terrorizing and another count of violating conditions of release. Those charges were dismissed in exchange for the plea.