ANCHORAGE, Alaska — A private dive team has discovered the wreckage of an American ship that sank off the south-central Alaska coast 139 years ago.

The Torrent sank in Cook Inlet in 1868 after tidal currents rammed it into a reef south of the Kenai Peninsula. Documents from the period show that all 155 people on board survived.

America had purchased Alaska from Russia less than a year earlier, and about 130 Army soldiers had come north on the Torrent to build the first American military fort in south-central Alaska.

The shipwreck is the oldest American wreck ever found in Alaska.

“It’s a very significant find because it’s right after the purchase, during the transition from Russian to American authority,” said Judy Bittner, a state historic preservation officer. “It’s the very beginning of federal presence in Alaska and the establishment of order.”

A four-man dive team led by Steve Lloyd, owner of Anchorage’s largest independent book store, found remnants of the wreckage in July. They kept the discovery secret at the request of state officials, who wanted more time to document the site before any looters arrive. Its discovery was announced Monday.

An array of objects, from guns, cannons, shoes, and plates, are hidden beneath the broad leaves of giant kelp beds or concealed in caverns and crevices among massive boulders, Mr. Lloyd said.

“It’s like walking through a field of tall grass and undergrowth looking for a baseball that you’ve lost,” Mr. Lloyd said.

Big finds include the two anchors, sections of hull and heavy bronze rudder hinges weighing about 100 pounds.

About 2,500 ships have wrecked off the Alaska coast since Russian explorers first arrived in 1741, according to Mike Burwell, a cultural anthropologist for the federal Minerals Management Service. A partial database on the service’s Web site lists Japanese submarines and fishing trawlers, Liberian freighters, and New England whaling ships, among others.