The man arrested in Rome today after the attack on Pope John Paul II is believed by Turkish authorities to be Mehmet Ali Agca, the convicted slayer of a prominent Turkish journalist and a man who allegedly threatened the pontiff during his November 1979 visit to Turkey.

[In a news conference in Washington, Turkish Ambassador Sukru Elekdag said he had no direct word from his government or from Italy that the man being held was actually Agca, but he noted news reports suggesting that Agca was in custody and called him "a well-known fugitive from Turkish justice. . . . Turkish police are under instructions to shoot him on sight."]

In February 1979 Abdi Ipekci, the moderate editor of the daily Milliyet, was slain, and Agca, 24, was arrested five months later. He told reporters he was not affiliated with any group on the left or right and that he killed Ipekci "because he supported exploitation."

Agca escaped from the maximum security Kartel military jail in Istanbul a few days before the pope's scheduled visit in November of that year. Law enforcement authorities here believe that Agca is affiliated with an extreme right-wing terror organization known as the "Gray Wolves," which is associated with the Nationalist Action Party of Alparslan Turkes.

After he escaped, a letter allegedly from Agca was published in Milliyet that said, "The only reason I escaped from jail is to kill the pope."

The letter, which was deposited in a trash can outside a drugstore in front of the newspaper's offices, said, "Western imperialists, who fear that Turkey may establish a new political, military and economic power in the Middle East with brotherly Islamic countries, have at a sensitive time, quickly sent crusader commander-in-chief John, who wears the mask of a religous leader, to Turkey.

"If this untimely and meaningless visit is not canceled," the letter continued, "I will definitely shoot the pope. This is the only reason why I escaped from jail."

[The Italian news agency ANSA said police in Rome found a letter written in Turkish on the suspect in which he allegedly said the attack on the pontiff was meant "to demonstrate to the world the imperialistic crimes committed by the Soviet Union and the United States."]

[ANSA also reported that seven days before the attempt on the pope's life, the turkish police had put out an interpol bulletin warning of the man's arrival in Italy. ANSA said Interpol warned that he had been convicted of killing the Turkish editor.]

Agca, a tall gaunt young man with prominent cheek bones and black hair, was born in the eastern town of Malatya to poor parents. The military authorities instituted proceedings to strip him of his nationality after he ignored calls to turn himself in.

Turkish newspapers reported that several months after escaping, Agca went to West Germany, where he married a German woman to prevent his extradition to Turkey. A Turkish Cabinet minister said that Turkey had asked international police organization Interpol to assist in his capture. h

Despite the religious references in the alleged threatening letter from Agca, his identification with the ultra-rightist party does not suggest that he is connected with the religious Islamic fundamentalists in Turkey. Rather, the Gray Wolves organization is known for a brand of extreme rightwing nationalism. Its followers blame the violence and disintegration of authority in Turkey on the left and say they organized as a paramilitary group to combat left-wing extremism.

Before World War I, Turkey, as the Ottoman Empire, was one of the major Moslem powers in the world. Modern Turkey was established as a secular republic in 1923 under reforms carried out by Kemal Ataturk. The basic thrust of these reforms has been to modernize the state and bring it closer to Western Europe, and away from the Islamic East. Although there has been some revival of Islamic fundamentalism in Turkey, which is not an Arab country, since 1973, election results indicatde that this has affected less than 15 percent of the population.

The Gray Wolves organization has been blamed for hundreds of killings in the terror-filled period before last September's military takeover in Turkey. The group is named after the legendary beast that led the Turks from Central Asia to their present homeland in Asia Minor.

The modern Gray Wolves were originally young men trained in armed combat and indoctrinated with party nationalism in secret camps in the countryside, then unleashed onto university campuses to counter extreme left-wing influences. What started as a battle for the domination of the campuses turned into a struggle for the control of the country.

Turkes' Nationalist Action Party gained some political respectability in the mid-1970s, when Turkes was appointed deputy prime minister in the coalition government of Suleyman Demirel. In 1977, the party gained 16 seats in the 450-seat National Assembly to become the fourth-largest party.

Since the military takeover, however Turkes and about 220 of his followers have been on trial on capital charges of instigating civil war in Turkey.