Where is Abu Nidal? News reports say the notorious terrorist has been detained in Egypt. Egypt denies it. A onetime guerrilla colleague, confronted with the question, replies with one of his own: Which Abu Nidal?

The comment says much about Abu Nidal - Arabic for "father of struggle" - whom the United States once labeled the world's most dangerous terrorist. Over the years Abu Nidal has been reported to have died - but then suddenly reappeared - or to have totally changed his looks through plastic surgery.Rumors have been circulating in the Middle East for about two weeks that the one-time Palestinian guerrilla was arrested by Egyptian police while coming from Libya and is either jailed or hospitalized with leukemia.

Wednesday, Foreign Minister Amr Moussa denied that Abu Nidal was in his country when asked by reporters about the American media reports. U.S. officials in Washington said they could not confirm the reported arrest.

The reports persist because of Abu Nidal's notoriety. His group was once listed by U.S. State Department as "the most dangerous terrorist organization." He has been blamed for killing 300 people in 20 countries since 1973.

Palestinian officials have told The Associated Press that Abu Nidal - whose real name is Sabri al-Banna - is either jailed or hospitalized in Egypt.

The latest reports quoted U.S. or Arab officials, and one from ABC News quoted unnamed Egyptian authorities. But no one has offered definite proof that Abu Nidal is indeed in custody in Egypt.

One high-ranking Palestinian, who spoke on condition of anonymity, said Wednesday that Abu Nidal was seized by the Egyptians about five weeks ago after coming from Libya.

He said he was uncertain whether Libya arranged for the Palestinian guerrilla to leave for medical reasons or if Abu Nidal's own people were trying to smuggle him out through Egypt to another country.

Abu Nidal is near death and, though he is being treated in a hospital, he is in detention, the official contended.

He and others theorize that Egypt is denying the arrest because it does not know what to do with the 61-year-old guerrilla and fears revealing his arrest could make Egypt the target of other terrorists.

Born in Jaffa in then-Palestine in 1937, Abu Nidal's main targets have been Israelis. But he also was blamed for killing some of Yasser Arafat's chief aides after he split with the PLO leader, claiming Arafat was not keeping up the struggle against Israel.

Over the years, Abu Nidal is said to have operated out of Iraq, Syria and Libya.

The terrorist leader is not wanted in the United States, although he has been blamed for blowing up two American airliners: a Pan Am jet on the ground in Rome in 1973 and a TWA plane in 1974 in the air over the Aegean Sea. The combined death toll was 120.

It was a Palestinian in Lebanon who asked the question about "which Abu Nidal?" - raising the theory that the terrorist actually is dead and that some followers have used his nom de guerre. The Palestinian, once close to Abu Nidal's group, spoke on condition of anonymity.

Over the years, the chain-smoking Abu Nidal has been reported to be dead of cancer or a heart attack. After reports of his death in the mid-1980s, he was quoted in one interview as saying:

"I'm neither a ghost nor a phantom. I am Abu Nidal of flesh and blood . . . this evil spirit which moves around only at night . . . causing these people constant nightmares."