The Coast Guard is looking for a diver last seen Thursday afternoon inside the Spiegel Grove wreck, a Navy ship sunk in 2002 as an artificial reef offshore of Key Largo.
Joseph Dragojevich, 43, of Orlando, was last seen during a dive inside the wreck with James Dorminy, 51, of Kissimmee. The two were on a commercial dive, and they decided to enter the 510-foot-long ship while the others did not, said Monroe County Sheriff's Office spokeswoman Becky Herrin.
The men attached a reel line when they entered so that they would be able to find their way out.
Dorminy told deputies that they explored several levels of the ship before beginning their exit. Dorminy said he was in the lead and Dragojevich was reeling in the line behind him.
When Dorminy last saw him, Dragojevich signaled with his dive light that he was all right, Herrin said. But when Dorminy turned back again, Dragojevich was gone and the line was slack and tangled.
Dorminy searched for Dragojevich until he started to run out of air and then surfaced for help, Herrin said. An initial search by the Coast Guard and state wildlife officers failed to find Dragojevich.
The sheriff's office and divers from the Key Largo Fire Department continued to search the wreck Friday.
The Spiegel Grove sank prematurely off Key Largo in July 2002, leaving the ship's bow sticking out of the water for several weeks.
The Spiegel Grove is the world's largest artificial reef. Six people have died while diving inside the reef since 2002.