Two wells at the site of a planned 20-unit rental development in Tisbury were purposely contaminated with motor oil, Philippe Jordi, executive director of the Island Housing Trust said Thursday.

The incident is under investigation by the Attorney General’s office and Massachusetts Environmental Police, and the IHT board of directors has offered a $5,000 reward for information leading to the conviction of the person or persons who tampered with the wells, Mr. Jordi said.

The wells have been plugged, capped and decommissioned by environmental engineers from the IHT and the Massachusetts Department of Environmental Protection to remediate the situation, he said.

“We’re pretty confident that the aquifer has not been contaminated, but it’s pretty disturbing,” Mr. Jordi said. “It’s not only jeopardizing our property, but neighboring properties as well.”

The two wells were drilled in January to serve as the public water supply for the Kuehn’s Way development, a neighborhood of 20 rental apartments scheduled to begin construction this spring. The project is located off State Road in Tisbury across from the Assembly of God church.

The first well was found to be contaminated with motor oil when required state environment testing was conducted on Jan. 26, Mr. Jordi said. The second well tested clean at that time, but was found to be contaminated in subsequent testing done this week, he said.

Tisbury police and board of health were immediately notified and the case is under investigation. Anyone who may have information about the crime are asked to contact Detective Lieberwirth of the Massachusetts Environmental Police at 617-963-2657.

A news release issued by IHT said underground monitoring wells have been installed near the tampered wells and abutting properties for ongoing water quality testing.

Mr. Jordi said the trust is securing the site with fencing and additional protections and will be drilling two new wells. Though the contamination will add unexpected costs, he said it has not altered the timeline for construction of the Kuehn’s Way neighborhood, which is scheduled to open in early 2022.