Investigators trying to track down the source of an outbreak of gastrointestinal illnesses on a Lake Erie resort island are focusing on drinking water.

Ohio Environmental Protection Agency officials ordered inspections yesterday on private wells to determine whether they are connected to and contaminating the village of Put-in-Bay’s drinking water system.

The concern is that leaking septic tanks could be contaminating well water and then flowing back into the clean drinking water.

There’s no evidence yet that is happening, said Heidi Griesmer, an EPA spokeswoman, who called the order a precautionary measure.

“We know what’s coming out of the plant is meeting water quality standards,” she said. “We don’t know if there’s contamination being introduced somewhere in the distribution system.”

The Ohio Department of Health said yesterday that investigators have talked with 1,020 people who say they fell ill after visiting South Bass Island and the surrounding area, which is about halfway between Toledo and Cleveland.

All but a few visited the island. Some say they were sickened after visiting within the last week. About 40 people spent time in a hospital, the health department said.

The department has tested a handful of samples from those who say they suffered from chills, fever, diarrhea and vomiting. Eighteen people tested positive for one of two types of bacterial infections or one type of viral infection.

Those test results led investigators to begin focusing on whether there could be any cross-contamination between private wells and wastewater systems.

“That would give us an indication that this cross-contamination issue is what’s going on here,” said Jay Carey, an Ohio Department of Health spokesman. “We don’t know that for sure.”

Teams of investigators have been testing water samples from both homes and businesses on the island.

The EPA ordered four businesses yesterday to stop using its water after tests found that wells at those businesses contained E. Coli or bacteria, Griesmer said.

Well water at one of the businesses — an island winery — earlier tested positive for E. coli. A second test last week found no bacteria.

No one, though, has tested positive for E. coli.

In addition to inspections of auxiliary wells, which island residents often use for watering lawns or washing cars, the EPA asked for increased monitoring of private water supplies.

The health department recommended island residents use bottled water or boil their well water.

Nearly all the businesses on the island get their water from the village’s water system.

Put-in-Bay Mayor Mack McCann has said the village’s water system has been tested regularly with no negative results.

Businesses on the island, a popular summer getaway filled with bars, restaurants and charming inns, has taken a hit in the last two weeks.

Most people said they became sick within two or three days of visiting the island. Symptoms have generally lasted about 24 hours.