Sir Gruff, a wild horse, grazes at Assateague Island. He was struck and killed by a car Tuesday night. (National Park Service Assateague Island)

He was known as Sir Gruff.

On Tuesday night, the 18-year-old solid bay stallion was struck and killed by a car on Assateague Island, officials said.

Officials with the National Park Service said Sir Gruff is the 19th wild horse since 1982 to be struck and killed along State Highway 611, the main road into Assateague on the Delmarva coast. Six horses have been injured during the same time period along the road.

There is an investigation underway into the crash involving Sir Gruff. But at this time, “it is not known if speed was a factor,” according to a Facebook post from the National Park Service.

Last year, another wild horse — an 18-year-old stallion named Jester — was struck and killed by a car.

Sir Gruff and his band of wild horses, including two named Ninka and Assateague Phoenix, typically roam in the areas of the Tingles Island and Pine Tree backcountry, officials said. In the summer, they were known to move in the more developed area of the island where there’s the road, a campground and a parking area.

The crash, Park Service officials said, happened in the developed area about 7:30 p.m.

Park Service officials said the wild horses at the island are “a treasured part of the Assateague experience.”

They said they’re concerned about the “loss of horses on the causeway” and are working with Maryland highway officials to try to make Route 611 “safer for visitors and wild horses.”

Assateague is known for its free-ranging herd of wild horses. Along the main highway into the park, there are signs reminding visitors to slow down and look for wildlife. The Assateague and Chincoteague islands are known for an annual pony swim in the summer. Dozens of wild ponies swim from Assateague Island to Chincoteague Island in Virginia.

The swim draws thousands of visitors from across the country. It grew in popularity over the years and became well-known because of Marguerite Henry’s 1947 novel “Misty of Chincoteague,” which later became a movie.