The bacterial contamination in the Oak Bluffs water supply last month that forced town residents to boil their tap water or use bottled water for nearly a week was likely caused by a private contractor who broke through a water line, town water officials said this week.

The identity of the contractor may never be known, officials said. A notice published in today’s Gazette by the Oak Bluffs Water District states that the source of the coliform bacteria found in the town water last month remains under investigation, but the cause was likely a contractor.

“All indications are directed toward several water services broken by a private contractor,” the notice says. “The district feels that reversed flows caused by these breaks, the subsequent repairs, and the decrease in flow in the system from summer to fall, may have been contributing factors.”

On Sept. 25 the state Department of Environmental Protection issued a boil-water order for the town after coliform bacteria was found in several water samples during routine monthly testing. Customers of the town water district, which includes most of the town, were told to use bottled or boiled water for nearly all their needs.

The DEP lifted the order six days later, after the town passed three consecutive days of water testing. Many residents complained they did not learn of the order until several days after the fact, and questions surfaced about the town’s ability to get information to the public during an emergency.

For a full week the town established an emergency dispensing site at the public library on Pacific avenue and gave away approximately $36,000 worth of bottled water to residents.

Water district superintendent Thomas Degnan this week said it is not uncommon for a private contractor to accidentally break through a water line with a backhoe or drilling device; when this occurs the line is shut down and repairs are made.

Mr. Degnan said the water district responded to several breaks in the water lines before the state order was issued; he said it is would be hard to pinpoint which one of the breaks led to the problem.

“We know there were many construction projects in the north end of town, and we had another hit from a state construction crew doing some new storm drain installation . . . but right now it’s just a bunch of assumptions with facts. We may never know the actual source,” Mr. Degnan said.

The public notice says routine water testing that took place from Sept. 22 to Sept. 24 revealed that coliform bacteria was found in 41 of 62 samples collected. Coliform bacteria are naturally present in the environment and are used as an indicator that other, potentially harmful bacteria may be present.

“As our customers, you have a right to know what happened and what we did to correct the situation,” the notice states. “The district is reviewing treatment plant operating procedures and other factors that may contribute to coliform outbreaks to insure that the possibility of a reoccurrence is minimized, if not eliminated.”

Selectmen this week unanimously voted to retroactively declare a state of emergency for the week of the water crisis, in order to pay the vendors who provided the water given out to town residents. The town will pay $33,639 to Poland Spring Distributors in Barnstable, and $2,530 to Chilmark Spring Water Co. Additional payments for staffing bring the total bill for the week to just over $39,000.

Selectman Ron DiOrio said the water district, which has its own governing body and functions as an entity distinct from the town government, should eventually foot the bill.

“My position is clear. It in reality is the responsibility of the water board to incur the cost of this emergency. I don’t want any vendor to be kept waiting, so I am going to support this motion so they can be paid. But in fact I believe the town has to be reimbursed by the water board,” he said.