For Coast Guard Cmdr. D.C. Addison, the search for the "mystery hero" of the Air Florida crash was a three-month wade through hundreds of documents from half a dozen government agencies for "that one little bit of pertinent data" that might pin a name to the haunting image of the man who repeatedly passed a helicopter lifeline to others.

In the end, Addison said, "nothing by itself was that convincing."

But, he said, the accumulated weight of all the data "convinced us absolutely" that Arland D. Williams Jr. of Atlanta was the man in question.

Addison, who regularly evaluates the Coast Guard's rescue operations, had questioned survivors and those involved in their rescue shortly after the Jan. 13, 1982, accident in an effort to certify those deserving lifesaving awards.

He ended up, he said, recommending awards for the two U.S. park policemen piloting the rescue helicopter, for Lenny Skutnik and Roger Olian, two bystanders who swam through the ice-clogged Potomac River to the victims' aid, and for the unknown survivor who passed the helicopter's life ring to others, giving up his own chance to be rescued.

Williams' mother, Virginia Williams of Mattoon, Ill., wrote President Reagan, noting media speculation that her son, a 46-year-old bank examiner for the Federal Reserve System, was the man in question and asking that, if he was, he be referred to by his name rather than as "the mystery hero."

As a result of that letter, Addison said, the Coast Guard reopened its investigation last November, using information unavailable immediately after the crash.

The most crucial information, he said, were autopsy reports, photos of the recovered remains and interviews with divers who retrieved the remains from the Potomac and thus knew their position in the wreckage beneath the ice.

Though photographs of Williams did not match descriptions of the "mystery hero" given by survivors, Addison said, almost all the victims died of injuries too traumatic for them to have survived the crash. Williams died of exposure and drowning.

Addison presented his findings to the Coast Guard Board of Awards, which made the final determination in the case and awarded Williams the service's Gold Lifesaving Medal.