It's just an old steel gas tank, discreetly tucked behind two storage trailers on a construction site in southern Fairfax County. But when workers from Heritage Site Development went to gas up their vehicles Tuesday morning, it was empty. Four hundred gallons of off-road diesel fuel -- gone.

Mike Bertelson, Heritage's owner, said the fuel cost him about $1,000, at "only" $2.50 a gallon. "That's a lot of money, for me," Bertelson said. "My fear is it's going to become more rampant."

As gas prices have soared, first with rising crude oil costs and then the disruptions of Hurricane Katrina, gasoline has become a far more precious commodity, inspiring increasing thefts at the pumps and more siphoning from motorists' gas tanks, authorities said yesterday.

Before Katrina hit, the Maryland State Police barracks in La Plata typically received two reports a week of motorists pumping gas and not paying. Two weeks ago, the call volume climbed to at least two a day and sometimes more, said Detective Sgt. Ronald Best, who responded to the calls.

"It dramatically changed after the hurricane," he said. "Gas is almost like liquid gold."

The rise in the number of motorists driving off without paying persuaded the owners of at least one chain of convenience stores to change the way customers pay to fill up. Last Saturday, Wawa stores began requiring drivers using cash to pay in advance.

"Industry-wide, gasoline retailers are experiencing a marked increase in 'drive-offs' at fuel pumps," said a statement released by Pennsylvania-based Wawa to explain its prepayment policy. "Drive-offs may create unsafe situations on store lots, and contribute to higher prices."

After the policy changed this week, Best said he noticed a drop in the number of calls from gas stations.

Jeff Lenard, a spokesman for the National Association of Convenience Stores, said one store in the Midwest reported three thefts in one day totaling $210. "You can't afford even one of those a day with the low [profit] margins," Lenard said. "We're seeing a dramatic increase in the number of retailers who say, 'That's it, we're going to go prepay,' " Lenard added, although that discourages some customers from stopping.

The District has the most-expensive gas in the country, according to AAA Mid-Atlantic, at an average of $3.34 a gallon for regular unleaded, while Maryland's gas averages $3.23 a gallon and Virginia's averages $3.09. A D.C. police spokesman said there had been no reports of gasoline thefts in the District.

In Waldorf, library technician Patricia Baker returned home from browsing yard sales last weekend to find the gas tank of her Chevy Cavalier drained.

"If someone needed some, I would have given them half of what I had, but to take it from you like that . . . " she said in disbelief.

Baker, 64, remembers waiting in long lines to buy gas during the Arab oil embargo in the 1970s. But, she said, "they didn't steal your gas then." Baker said she went out Thursday to buy a gas cap with a lock.

Fairfax police had four reported instances of gas siphoned from private vehicles in the first three days of this week, Officer Beth Funston said. "Since the price of gas has gone up, we've noticed an increase in these issues," she said.

The 400-gallon theft from the construction site was also in Fairfax -- on Gunston Cove Road in the Lorton area. Funston said Heritage's tank was padlocked, and when workers arrived Tuesday after the Labor Day weekend, the lock was gone -- along with the fuel.

"I've got a mechanic working on some kind of device to make the lock tougher," Bertelson said. "But like they say, locks are only there to keep the honest man out."

Construction companies have not been reporting an increase in filched fuel, said Calli Schmidt, a spokeswoman for the Northern Virginia Building Industry Association.

Then again, she said, construction sites are perpetually beleaguered by thefts, gas being just one target.

"Short of hiring a full-time armed guard 24 hours a day, seven days a week, there's not a whole lot you can do," she said.

In Montgomery County, a police spokesman said the department is more concerned about residents who leave their garage doors open or sheds unlocked, exposing gas-guzzling lawn mowers and cars.

"That's just too tempting. Locking it up and putting it out of sight is important to prevent yourself from becoming a victim," said Officer Derek Baliles, a spokesman for the department.

"We definitely have seen an increase in drive-offs" in recent weeks, said Kraig Troxell, a spokesman for the Loudoun County sheriff's office. Between Sept. 1 and yesterday, the sheriff's office took eight reports of gas-pump theft. During the same period last year, four were reported, Troxell said.

"You can't get into their minds, but you would have to speculate that the higher prices . . . have something to do with it," Troxell said.

Shirley Floyd, a clerk at the Plaza Shell station in Leesburg, said surging prices have put the station on guard. These days, customers using cash after 6 p.m. are informed by notes on the pumps, and occasionally by a clerk's voice over the loudspeaker, that they must prepay.

"We just know that there's a higher potential for people" to pilfer fuel when prices are high, she said.

At a Shell station on Lorton Road in southern Fairfax, cashiers won't turn the pump on now for a customer if they are unfamiliar or have out-of-state tags, cashier Muhammad Butt said.

"We want them to come in, pay first," he said. "Then we turn them on."

Staff writer Karin Brulliard contributed to this report.