A Yonkers couple was arrested Monday after an off-duty Suffolk police officer saw them stealing used cooking oil from behind an Islandia restaurant and matched their vehicle with a van sought in a rash of similar thefts, officials said.

Police said the couple, Domingo Santos, 40, and his girlfriend, Vanessa Liriano, 31, were in possession of about 650 gallons of used cooking oil, which was being stored for resale and refinement into biofuel. The oil had a resale value of about $2,000, police said.

The investigation is continuing, police said.

Santos and Liriano were each charged with 16 counts of petty larceny, one count of possession of stolen property and one count of possession of burglar's tools, Suffolk County police said. Santos also was charged with one count of criminal possession of marijuana.

Police said the incident took place behind the New Grand Buffet on Veterans Memorial Highway at about 8 a.m.

The off-duty Fourth Precinct Crime Section officer was on his way to work when he spotted the couple's 2004 Ford van and noticed it fit the description of a van being sought in connection with thefts of used cooking oil from restaurants.

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The officer, whose identity was not released, witnessed Santos "withdrawing used cooking oil from an oil canister located behind the restaurant," police said in a statement. The officer then called to report the theft.

The couple was arrested by responding Fourth Precinct officers, police said.

Santos and Liriano were arraigned Monday in First District Court in Central Islip.

Bail was set at $23,000 for Santos and $22,000 for Liriano. It was not clear Monday afternoon whether Santos had an attorney; Liriano was represented by Legal Aid, which has a policy of not commenting on cases.

With Ellen Yan

CORRECTION: An earlier version of this story gave the incorrect location of the restaurant.