State officials Thursday said they are investigating the circumstances that led to significant coral damage near the entrance channel to Honolulu Harbor after dredging work done by a contractor.

The state said the U.S. Army Corps of Engineers hired Healy Tibbitts Builders Inc. for dredging work at the harbor.

Healy Tibbitts allegedly dragged the dredging platform’s anchor and cable across several coral colonies — toppling one estimated to be several hundred years old — and deposited debris on top of them, according to the state Department of Land and Natural Resources.

“This is a large and significant damage event,” said state Division of Aquatic Resources Administrator Brian Neilson in a news release. “The last few days our dive teams have been in the water doing further assessments and documenting the scope of the damage.”

Divers from the U.S. Fish and Wildlife Service who were conducting water quality monitoring first noticed the damage May 4 and reported it to state officials.

They observed and photographed damage stretching across an estimated 17,500 square yards that included both broken and toppled colonies as well as coral smothered by dredged material.

“We’re already seeing mortality among some of the corals that were damaged,” said David Gulko, manager of the Anuenue Coral Restoration Nursery at Sand Island. “Time is of the essence, with the summer’s first south swells imminent, so we need to fix as much damage as we can, quickly.”

Among the corals damaged is a significantly large Porites evermanni colony estimated to be hundreds of years old.

Gulko said given its size of about 2 meters in diameter, and that it grows only about 1 to 2 centimeters a year, it is likely several hundreds of years old. The species is endemic, meaning only found in Hawaii.

Divers found this previously healthy colony of live coral overturned, on the bottom and in danger of dying due to suffocation, officials said. Saving this colony will require specialized equipment to return it to an upright position as soon as possible to increase its chances of survival.

If action is taken quickly, Gulko said, portions of it could still survive and recover.

“A coral that big and that old is extremely valuable in terms of the ecological serv­ices and functions it provides to the environment and to the people,” he said.

State divers today will attempt to reattach broken corals using a special marine epoxy, and collect other fragments for the coral restoration nursery, where specialists hope to grow them out for eventual return to the ocean.

The documented damage may be presented to the Board of Land and Natural Resources, which has the authority to impose fines on responsible parties to pay for resources damage and remediation efforts.

“Healy Tibbitts understands the significance of Hawaii’s precious natural resources, and we are committed to doing everything we can to take immediate restorative action and mitigate further damage to the coral reef,” said Emmett Kinney, vice president of Healy Tibbitts Builders Inc., in a statement. “To that end, we have retained help from Marine Research Consultants, Inc. and Sea Engineering, Inc. to rapidly commence mitigation and restoration efforts.”

Healy Tibbitts is developing and submitting a dive plan and restoration plan, and obtaining a special-activity permit to take restorative measures.

“(Healy Tibbitts Builders) is making every effort to respond rapidly and take swift restorative action,” said Steven Dollar, president of Marine Research Consultants. “Their collaborative efforts — partnering with our team and Sea Engineering, Inc. — will ensure that they have the best technology and our deep experience studying and restoring Hawaii’s coral reefs.”

According to DLNR, this is one of the most significant incidents of damage done to Hawaii reefs since 2009, when the USS Port Royal, a Navy missile cruiser, ran aground and damaged the coral reef within sight of Hono­lulu International Airport.

The Navy agreed to pay $8.5 million to the state for that incident, DLNR said, the largest settlement for reef damage at that time. As part of remediation efforts, divers collected and reattached 5,400 loose coral colonies.