BRENTWOOD — A convicted police impersonator decided to plead guilty to charges he duped a Derry car dealer by using a stolen credit card to buy a truck that helped him pose as a U.S. marshal.

Eric Williams, 30, formerly of North Andover, Mass., received a 3½- to 7-year prison term on Thursday that will run concurrently with a 30-month sentence he is now serving in federal prison for false impersonation of a federal officer and mail theft.

Williams’ change of plea comes three days after a judge decided that a jury in his latest case would be able to hear about his past convictions for theft, forgery and receiving stolen property dating back to 2002.

The charges in Rockingham County Superior Court center around a series of events that happened on Dec. 23, 2010 — just before he began posing as a U.S. marshal and stole roughly $1,300 from two women.

Williams had stolen his landlord’s credit card, but to the dealership, it appeared he had permission to use it.

Prosecutors say a woman called posing as Williams’ mother saying her son had permission to use her card so he could buy a Ford Explorer they had for sale, prosecutors said.

Williams later had the truck outfitted with dashboard lights similar to those in law enforcement vehicles and bought a pellet gun that resembled a real firearm, according to federal court documents.

After those purchases, Williams had stolen and cashed two checks from a pair of Massachusetts women — one for $600 and another for $700 — while posing as a deputy with the U.S. Marshal Service between last Jan. 11 and Jan. 15.

His victims included a woman he dated for a brief time, and a roommate to another woman he dated briefly, according to court documents.

He pleaded guilty on Dec. 2 in U.S. District Court in Boston to seven counts of false impersonation of a federal officer and one count of theft of mail.

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