Nearly a year after Israeli spy Jonathan Jay Pollard was sentenced to life in prison, Justice Department officials have concluded that Israel almost certainly had another American espionage agent -- dubbed "Mr. X" by government investigators -- in the CIA or the Defense Department, according to informed sources.

The Justice Department is continuing its investigation to attempt to establish the identity of a second Israeli spy, the sources said.

The belief that a Mr. X must exist grew out of a massive, three-stage debriefing and polygraphing of Pollard over many months, the sources said. Pollard told U.S. officials that his Israeli handlers often specified by date and document control number the highly sensitive U.S. documents they wanted him to acquire, and once showed him a top secret U.S. document to which -- investigators have established -- he did not have access.

It was an index or catalogue that Pollard was then able to use to obtain documents from U.S. intelligence agencies during one phase of his espionage. U.S. investigators have concluded that the other American -- Mr. X -- had provided this index and other specific information on U.S. secrets that Pollard learned from his Israeli handlers.

U.S. intelligence agencies have drawn up lists of suspects, but no evidence has been developed so far pointing to any individual as a possible Mr. X.

Justice Department spokesman Terry H. Eastland said yesterday, "As we have indicated previously, there are unanswered questions in the case and the investigation continues."

An Israeli Embassy spokesman, Yossi Gal, said yesterday that the Pollard case was an aberration and the Israeli government has no spy or spies in the U.S. government and does not conduct espionage against the United States or its interests.

The alleged Mr. X, according to investigators, must have been so highly placed or in such a position that he could not regularly gather documents, whereas Pollard was a low-level analyst who had a courier card and whose job was to assemble information. Investigators concluded that Israel may have been more interested in protecting Mr. X than Pollard.

Efforts by the U.S. government to probe for the identity of Mr. X ran into a stone wall in Israel, where officials heatedly denied there was or had been another spy. One source who dealt with the Israelis in the investigation said, however, that "the denial was less than categorical."

U.S. investigators also have been searching for evidence of a direct connection between Pollard's arrest on Nov. 21, 1985, and the Israeli role in secret arms sales to Iran. The previous August and September, Israel had shipped 508 American-made TOW antitank missiles to Tehran with U.S. approval.

Within weeks of Pollard's arrest, Lt. Col. Oliver L. North, then an aide on the National Security Council staff, received Israeli permission to use $800,000 left over from an aborted Israeli sale of U.S. military spare parts to Iran for "whatever purpose we wanted," North has testified.

The money was used to buy military equipment for the Nicaraguan contra rebels, the first such diversion in the Iran-contra affair.

Investigators for the Justice Department and independent counsel Lawrence E. Walsh attach significance to the timing of the Pollard arrest and the first contra diversion. One key investigator said recently, "There has to be a connection . . . two of the most important events of 1985 to both {the} United States and Israeli governments weren't occurring in a vacuum."

Shimon Peres, then prime minister of Israel, has told associates that as a result of the Pollard case, he wanted to help win the freedom of U.S. hostages being held in Lebanon "to please George Shultz . . . and do a favor for America and maybe they would forgive us for Pollard," according to one informed Israeli source.

Pollard has a near-photographic memory, the sources said, and was able to recall documents, dates and technical material on a broad range of subjects. He pleaded guilty to espionage charges.

The three stages of Pollard's debriefing were the initial interrogation of some 30 to 40 hours; weeks of polygraph examination conducted by one of the Federal Bureau of Investigation's most experienced specialists, and in-depth debriefings by members of U.S. intelligence agencies lasting several hours three or four times a week over many months.

Justice Department investigators, including FBI officials and polygraph specialists, concluded that Pollard was being truthful in his post-sentencing debriefings as he explained meticulously how he was able to visit the Central Intelligence Agency, National Security Agency and other classified facilities to obtain documents from computers and files, the sources said. Investigators concluded that Pollard also was telling the truth when he said he was working alone and was not directly aware of another Israeli spy, though he explained in detail how the Israelis gave him top secret information about what documents and files they wanted him to gather.

The quantity and scope of the intelligence information that Pollard passed to Israeli officials during his 17 months of spying has yet to be explained in public.

As pieced together from intelligence sources it included:Technical and other information on special NSA projects designed to intercept foreign communications called signals intelligence (SIGINT), and to protect the security of U.S. military and intelligence communications. Sources said that in the government, NSA has claimed that it must replace the capabilities disclosed by Pollard, and that this will cost billions of dollars. But other officials have said this cost estimate is much too high. Code word and Sensitive Compartmented Information (SCI), the most important U.S. intelligence secrets, to which Pollard had access as an employee of the U.S. Navy's top secret antiterrorist effort. It was so broad and extensive that, in the words of one source, the espionage operation "allowed Israel to spy on every country in the world" by using the information gathered by all U.S. intelligence agencies. Included were not only intelligence on the Middle East but also vital, in-depth assessments of U.S., Soviet and other foreign military capabilities. One official referred to Pollard's access as "mind-boggling."

During the course of the two-year Justice Department inquiry, investigators have established that fund-raising efforts for Israel in the United States were often used to identify pro-Israelis in sensitive positions in the U.S. government.

One source said that it was possible that Israel had several additional sources of U.S. intelligence information and not just a single Mr. X, and at least one senior Justice Department official has expressed skepticism that another Israeli agent will ever be found. But after being briefed on the case last month, Attorney General Edwin Meese III ordered that the Pollard case be kept officially open so investigators can pursue their inquiry. Several internal Justice Department memos on the case refer only to "Mr. X."

Early in the investigation, when U.S. officials were not certain who recruited Pollard, they referred to that recruiter as Mr. X. That Mr. X was later identified as Israeli Brig. Gen. Aviem (Avi) Sella, who was indicted here last year on charges of recruiting Pollard and receiving U.S. documents from him. Sella is in Israel and is not expected to stand trial.

From June 1984 to November 1985, Pollard provided the Israelis with thousands of documents -- some 500,000 pages, sources said. At times the Israelis could not photocopy the material fast enough, investigators concluded. The apartment used as the photocopying center for the documents at 2939 Van Ness St. NW was given high security status by the Israelis, including "TEMPEST" electronic protections to ensure that the equipment inside did not radiate signals that could be picked up outside the apartment.

Rafi Eitan, a former official in Mossad, an Israeli intelligence service, headed a special unit called Lekem that ran and controlled Pollard. Eitan circulated summaries of the documents obtained from Pollard within the Israeli Defense Ministry and the intelligence agencies where "Rafi was considered a god, getting the most incredible material ever," one source said.

U.S. officials worried that the Soviets or other countries have agents in Israel who may have been able to obtain some of the sensitive material or at least detailed descriptions of it. Another concern is that some in the Israeli military or intelligence establishment could have traded some of the information with other countries, including the Soviet Union, for intelligence on neighboring Syria, which is considered the most serious and immediate threat to Israel.

One source said Pollard supplied intelligence that investigators believe was passed to South Africa, a country with which Israel has maintained close relations.

At the time he was spying for Israel, Pollard was working at the Navy's new Antiterrorism Alert Center (ATAC), which was created to ensure that the Navy and Marine Corps had access to all U.S. intelligence that could alert the services to an incident similar to the Beirut Marine barracks bombing in the fall of 1983 that killed 241 U.S. service personnel.

After the bombing and another bombing of the U.S. Embassy annex on Sept. 20, 1984, the United States discovered that there were clues or warnings in their intelligence files that had not reached the field in time, or which involved such sensitive communications intercepts that they would not automatically be distributed to the field.

ATAC was set up as an antiterrorist clearinghouse with the blessings of the White House, the CIA and the Navy, which afforded Pollard extraordinary access and cooperation from the entire intelligence bureaucracy. Pollard could request sensitive information that no one wanted to deny to the Navy because the ATAC mission was to save American lives, the sources said.

A top secret, 46-page description of the damage done by Pollard to U.S. intelligence sources and methods was submitted last year to U.S. District Court Judge Aubrey E. Robinson Jr. by then-Defense Secretary Caspar W. Weinberger. Though the prosecutors had asked only that Pollard be given a substantial prison term, Robinson last March 4 sentenced Pollard to life. One source said that Robinson was stunned that one spy could do such damage. Pollard's wife, Anne Henderson Pollard, received a five-year prison term for assisting him.

In a supplemental, unclassified sworn statement to Robinson, Weinberger said, "It is difficult for me, even in the so-called 'year of the spy,' to conceive of a greater harm to national security than that caused by the defendant in view of the breadth, the critical importance to the U.S., and the high sensitivity of the information he sold to Israel."

Weinberger said that Pollard "both damaged and destroyed policies and national assets which have taken many years, great effort and enormous national resources to secure."

Staff researcher Melissa Mathis contributed to this report.