For months, used cooking oil has been disappearing from containers behind Washington area restaurants. Thieves have been siphoning tens of thousands of gallons a month and reselling it, authorities say.

Arlington County police last week arrested two men at Ballston Common Mall they say were trying just such a scheme.

Cooking-oil theft is on the rise as the evolving biodiesel market has led to an increase in its value, according to Detective Crystal Nosal, an Arlington police spokeswoman.

“Police departments in the region are becoming more involved to remedy this problem,” Nosal said.

Industry insiders say the Ballston arrests mark a major breakthrough in combating an emerging form of commodity theft.

“It’s a brand-new crime,” said Steve Blankenship, regional manager of Charlottesville-based Greenlight Biofuels, which buys used oil from restaurants. “And it’s happening on an unbelievable scale.” He said the oil can sell on the street for as much as $4 per gallon.

Fa De Zheng, 36, of Oxon Hill and Ming Gang Lu, 38, of New York were charged with grand larceny and related charges on Oct. 7. Police say they were breaking into an oil container used by a client of Greenlight Biofuels.

At least four restaurants serviced by Greenlight Biofuels have been hit, police said.

In recent years, companies have been buying used oil and converting it into biodiesel fuel, a petroleum diesel alternative that can be used in diesel engines.

According to Blankenship, the oil can also be converted into animal feed. Firms in that business pay more for the oil than biodiesel firms, he said.

Blankenship said his company has been losing about 20,000 gallons a month, or 5 to 10 percent of its business. He said the thefts have been happening for years but have increased significantly in recent months. The company learns the oil is stolen when it goes to collect it.

Greenlight Biofuels hired private investigators and engaged several area police departments — some of which did not take him seriously at first, Blankenship said.

“We’ve been actively trying to solve this for some time now, and we recently got traction,” he said.

Blankenship said about 10 companies in the area buy used cooking oil.

Arlington police said Greenlight Biofuels contacted investigators in September but reported that thefts have been a major problem since March. Arlington detectives began watching the oil deposits and caught the thieves in the act last week, they said.

“This is a huge relief for us,” Blankenship said.