By FABIOLA SANTIAGO

The story was strange from the beginning: A Cuban man rescued by the Coast Guard on Sept. 25 from a disabled boat near the Florida Keys said he wanted to return to Cuba.

Now it's stranger still. It's top secret, in fact.

When he first arrived, immigration and Naturalization Service officials discussed the case openly. They stopped talking, however, once they saw his file.

Since then, the FBI has dropped into the Krome Avenue detention camp to talk to the bearded Cuban. Agents won't comment on their visit. And the State Department isn't talking at all.

All the Cuban says is that he wants to go back to running a Polynesian restaurant in Havana - a story that INS officials just don't believe.

Even so, he is to be released today from Krome, according to the INS.

For three weeks now, immigration officers have been trying to figure out just who is the Cuban who says he has never been to the United States, but whose business cards are in English and whose clothes were made in America.

His file is kept in a safe at Krome, where only personnel with top security clearance from the government are allowed to see it.

"His file is classified," Immigration and Naturalization Service Deputy District Director Dwayne Peterson said.

According to what INS official can say about the file, the 39-year-old man actually entered the United States through Guantanamo in 1973, identifying himself as William Prado Matos. A federal source said the 12-year-old file was stamped "politically suspicious" and put away.

But the man at Krome insists that he shouldn't have a record in this country at all.

He says he is William Matos Prado, the manager of Havana's Restaurante Polinesio, a Polynesian-style eatery in the touristy Havana Libre complex. And despite the old INS file, he says he has never before been in the United States.

Matos isn't talking anymore. He declined to be interviewed by The Herald.

INS agents first grew suspicious because his belongings didn't fit his story, according to Miami INS District Director Perry Rivkind.

The only document he carried was a card with a name, William Matos Prado, which he said was his, and an address he said was his home in Havana. But the card was in English.

His pants and shirts also had American labels.

INS computers revealed that a William Prado Matos, born Sept. 20, 1946, had come to the United States in 1973. His status was indefinite "parole," and his file was stored in Boston.

Under questioning about the discrepancies in his identity, "he got a little belligerent," Rivkind said. "He appears well-educated."

For days, INS officials themselves weren't sure of the Cuban's identity, they said.

The pictures in the file and the man at Krome were not easy to match, they said. The clean-shaven. Prado in the files had a scar on his chin: The Matos at Krome has a beard, and INS hasn't asked him to shave.

"I don't think we have the legal right to do that," Assistant District Director George Waldroup said.

Still suspicious, INS compared the signatures and fingerprints from in the file.

"We are convinced," said Peterson, now certain the man was lying. Apparently, though, they are still uncertain why.