The 18 1/2-month search for convicted murderer Mehmet Ali Agca, now held in Rome for Wednesday's shooting of Pope John Paul II, underscores the problems facing law-enforcement authorities in the fight against international terrorism.

Law enforcement officials in Washington and Europe point out that there are many known terrorists who are hard to track down because of forged documents, ease of travel and the terrorists' underground life style.

Despite the fact that Agca was known at least by name in dozens of countries, officials were not surprised that he eluded arrest.

"There are hundreds of wanted terrorists in Europe," said Anthony C.E. Quainton, director of the State Department's Office for Combating Terrorism -- citing Italy's Red Brigades, French Corsicans, Spanish Basques, the West German Baader-Meinhof gang -- "who have been arrested at some time, managed to escape, being looked for by the authorities of many countries.

"You have to remember that for most of this time he [Agca] has been leading a clandestine or quasiclandestine life," and as such would be hard to track down in any society.

Agca, the apparent right-wing terrorist accused of trying to kill the pontiff with a 9 mm pistol in the Vatican yesterday, was the subject of a file sent to the international police organization Interpol early last year after he escaped from a Turkish military prison.

According to news service reports from Rome, police believe Agca had planned the attack on the pope for some time.

Turkish authorities have said Agca authored a letter to the Istanbul daily Milliyet in November 1979 threatened to kill the pope on his visit to Turkey that month.

He first entered Italy on April 9 with a false passport naming him as Faruk Azgun. He used that name to enroll at the University of Perugia, north of Rome, attending classes for one day before disappearing. Before that, he apparently lived in the rightwing underground in West Germany where there are at least 1.2 million Turkish migrant workers.

He apparently left Italy for a period, but reentered from Majorca, Spain, on the same false passport on May 9 and took a room in a cheap hotel near the Vatican.

In Paris, Interpol Commander Ray Kendall was quoted by the Manchester Guardian as saying a wanted notice was circulated to Interpol's 130 member nations immediately after Agca's file was received from Turkish authorities.

There was one unconfirmed report that Interpol warned Italian authorities a week ago that Agca was headed for Rome.

The Guardian reported that the wanted notice carried no warning from Turkish police about Agca's apparent obsession with killing the pope.

Kendall said Interpol issues about 200 wanted notices a year at the request of member forces, but the organization has no investigative powers, acting mainly as a secretariat and an international relay station.

Interpol would only add to a notice any details that had been transmitted previously, Kendall added.

"It is rare for a wanted man to be picked up as a result of routine checks at airports if he is traveling under a false name for the simple reason that there are too many people passing through," Kendall said.

"Most arrests are made as a result of routine police checks when the photograph and fingerprints provide the identity of the man on the run.

"The quick identification of the man alleged to have attacked the pope was probably based on the details supplied through us to the Italian police by the Turkish police. Any other contacts between the two forces would be on a bilateral basis."

The State Department's Quainton said one of the crucial difficulties in the hunt for any terrorist is the verification of identity.

Many terrorists travel on forged passports, and immigration officials are trained to try to recognize doctored documents.

"If you would find a particular name, you could put it into a computer at the port of entry, quickly, and get an alert, say 'Here's terrorist X or Y.' That depends on having a name in the system," Quainton said.

Because of the enormous volume of travelers and number of possible ports of entry, it is nearly impossible to rely on visual recognition of terrorists.