After President Reagan was shot in the March 30 assassination attempt here, the FBI took possession of a secret personal code card the president would use to authenticate his nuclear strike orders in certain military emergencies, according to government officials.

The loss of the card, taken as part of the evidence-gathering after the shooting, caused a serious dispute over its possession between the president's military aides and the FBI that was not resolved until Attorney General William French Smith ordered FBI agents at George Washington University Hospital to keep the card. It was not returned for two days. In the interim, officials say they presume a new card was issued and the codes were changed.

Knowledgeable officials say there was no loss of control over U.S. nuclear weapons, and that no crisis tested the system that day. The incident did, however, raise questions about how well the emergency system might have operated in time of crisis. In addition, the assassination attempt called into play a series of highly classified instructions relating to the delegation of Reagan's nuclear command authority.

The card is the only device personally carried by the president to authenticate his nuclear commands. It contains a series of coded numbers and words with which the commander in chief can identify himself to military officials in the war room at the Pentagon.

The code card was designed for use during emergency situations when the president might be without secure voice communications, such as the hour he spent in the hospital emergency room before undergoing surgery for his bullet wound.

The code and verification system is crucial to national security, since officials say they could have only a "handful of minutes" in which to make nuclear launch decisions in a crisis.

After Reagan's recovery the administration conducted a thorough inquiry into how the system worked that day. Reagan was reportedly disturbed by the discovery that the Joint Chiefs of Staff maintained a duplicate of the president's card without informing the White House. Reagan is said to have since taken some corrective action.

White House communication director David Gergen said Friday that this account contained some unspecified inaccuracies, then added, "At no time during the events of March 30 was the national security of the United States impaired. Beyond that, we have no comment."

Another knowledgeable administration official disputed this. This official, who declined to be identified, said, "A wounded president on its face jeopardizes the national security. But this system doesn't work, and the administration knows it has problems and there is no more crucial sensitive system that has to work."

Interviews with a number of knowledgeable officials also show how, in certain circumstances, "pre-delegation" agreements automatically relinquish the president's control over U.S. nuclear forces.

In the event of incapacitation or inaccessibility of the president, the authority to launch nuclear weapons can pass, according to officials knowledgeable with the agreements, through a chain of command that runs from Reagan to Vice President Bush to Defense Secretary Caspar W. Weinberger, to Deputy Defense Secretary Frank C. Carlucci, to Gen. David C. Jones, chairman of the Joint Chiefs of Staff. Each of these officials carries his unique code card which can, when command passes to him, be used to authenticate a nuclear order.

Authoritative officials say that while Reagan was under anesthesia for the four-hour operation that day, and later under heavy sedation for pain during his recovery, Bush would have held the president's nuclear proxy in the event of a crisis.

Since Bush was returning from Texas that afternoon aboard an Air Force jet that was not as well equipped as the president's plane for communications, some officials say Weinberger would have had the authority to make any decision to launch nuclear weapons in a crisis, since he is next in this line of command.

On his own authority, Weinberger raised the readiness of U.S. forces from condition five to condition four on the standard scale of five levels of alert.

Most officials agree that there was temporary disruption and confusion in the procedures for guarding the codes that activate the elaborate, top-secret cryptographic Sealed Authenticator System (SAS). The system is designed to guarantee proper control over nuclear weapons.

Other officials, all of whom asked not to be identified because of the highly sensitive nature of the system, said they are sure it provided for a quick change of codes in case of a security breach such as a lost card. These officials also said they believed that duplicate or alternate codes should have been available to the president through his military aides.

There is some question, however, over how quickly Reagan's military aides discovered that the code card was missing and attempted to recover it. It is not clear what fail-safe systems, if any, were initiated that day to provide the wounded and conscious president with alternate codes.

Reagan, like other presidents, has been carrying the card, usually in his wallet. The code card is changed periodically to prevent compromise.

In a military emergency it normally would be used by the president in a conference call with the secretary of defense, the chairman of the Joints Chiefs of Staff and the flag rank officer on duty at the National Military Command Center (NMCC), the secure war room at the Pentagon.

The military commander could simply ask the president to refer to various codes on the card that would match corresponding codes in the war room to ensure that the order was legitimate.

Though the Defense Department has taken the position that only the president can authorize the use of nuclear weapons, previous administrations have set up their own chains of command, officials say. If the president is unreachable due to health or communications problems, someone must hold his authority to respond to nuclear surprise attack.

The chain of command for emergency action is different from the line of succession specified by Congress and the 25th Amendment to the Constitution.

On the day of the shooting, it was apparently this line of succession that was confused in a public statement by Secretary of State Alexander M. Haig Jr. Officials say that delegation of the president's nuclear command authority historically has been an exclusive prerogative of the executive branch and that Congress apparently has not exercised oversight authority on the matter.

Television footage of the attack on Reagan at the Washington Hilton Hotel shows that the president and his military aide were separated immediately at the moment of the shooting. The aide, Lt. Col. Jose Muratti Jr., was shown on video tape running toward the motorcade as Reagan was pushed into his limousine. A White House official confirmed that Muratti did not ride to the hospital in the president's car, but followed in another car after an unspecified interval.

The importance of the aide, who is also known as the emergency war orders officer, is that he carries the top secret options and orders necessary for the president to choose a course of action in case of nuclear attack and translate that decision into military action. These orders are different from the code card carried by the president.

While dozens of people attended the wounded president at the hospital, the dispute over the president's nuclear code card, stripped away with his clothing, went largely unnoticed.

When Reagan collapsed at the hospital shortly after he stepped from his limousine at 2:35 p.m., he was carried and then lifted onto a stretcher in the emergency room. Nurses Kathy Paul and Wendy Koenig flanked the president.

Paul quickly sheared off the right front of Reagan's pin-striped suit jacket and monogrammed shirt with a pair of sharp surgical scissors she carried in her uniform pocket. Koenig struggled with a blood-pressure sling on the other side of the stretcher.

Paul came around to help. She ripped and cut through the fabric on his left side. Koenig got the blood-pressure sling attached. Almost in a matter of seconds, they had stripped the president. He was alert and talking as he lay on the remains of his jacket and shirt. Other shreds of his clothing dangled from the stretcher. A cuff link skidded across the polished floor, a tie tack dropped out of sight.

At first the clothes were ignored. "The suit jacket was left on the floor," said Drew Scheele, a general surgery intern who reached the president in the first few minutes. "It was literally underfoot while we worked on him, and was later kicked off to one side."

Scheele remembers this, he said, because a resident who was oblivious to the presidential scene passed near the trauma bay, spotted the torn clothing and remarked, "It must be an important guy, nice suit."

Reagan's belongings then were gathered up by D.C. police officials, Secret Service evidence technicians and, later, FBI agents, who by law have investigative jurisdiction over attacks on a president.

"I remember very clearly one Secret Service guy collecting all those belongings in a plastic bag," said David Gens, a surgery resident who later became one of two physicians attending Reagan during his recovery.

After 3 p.m. additional teams of Secret Service agents began arriving at the hospital to secure the various operating, recovery and intensive care rooms that would be needed by the president. At least four FBI agents arrived, including Robin Montgomery, a group supervisor in the political corruption section at FBI headquarters, Jack Oller from the FBI's Washington field office and James Werth, a counterintelligence agent detailed that day to catalog the evidence gathered at the hospital.

The president's stretcher and hand-carried fluid lines began a slow caravan to the operating room at 3:15 p.m. Evidence technicians followed behind, scooping up everything around the stretcher, including trash, linens and bits of clothing. As the president was prepared for anesthesia, about 3:30 p.m., the evidence-gatherers stationed themselves in an empty office near the operating room and began to catalog each item.

One law enforcement official who witnessed the handling of the president's belongings said that FBI agent Werth and others were in the midst of this process when they discovered they possessed the presidential code card. Sometime during surgery on the president, the military aide, Muratti, and Edward V. Hickey Jr., a White House security assistant, interrupted the FBI agents to ask for the return of the code card.

"Jim Werth was seizing all the evidence and they wanted that card," the law enforcement official said. Hickey reportedly insisted that the card be returned, and Werth left the office to make a telephone call.

The FBI officials were in direct contact with their superiors at the Washington field office. From there, the field office was in constant touch with the emergency command center at FBI headquarters, where senior aides to FBI Director William H. Webster, who was aboard an airplane, were monitoring the crisis.

The argument over the nuclear code card was flashed quickly to the FBI command center, where it was relayed to Attorney General Smith, stationed in the White House situation room with other members of the Cabinet. Smith instructed the FBI to keep control of the card, officials said.

Werth returned to the makeshift evidence room at the hospital and "let it be known that he had made his decision . . . that the card was secure . . . he had his orders and he was going to maintain control," the witnessing official said. One knowledgeable official harshly characterized this decision as an "inconceivable, myopic bureaucratic reaction." The official said the FBI and Justice officials should have recognized the sensitivity of all codes among the president's national security accessories. Additionally, the official says, the president's card had no connection with the shooting and no possible value as evidence.

Smith acknowledged the incident in a telephone interview last week, but would not discuss it. "I cannot comment in any respect. I can say that to my knowledge the procedures that were followed were all proper and they were taken intentionally and with a purpose."

The entire incident was kept from U.S. Attorney Charles F.C. Ruff, who is in charge of the investigation of the assassination attempt.

When Ruff asked questions about the matter recently he was informed that "There was a nuclear code that the FBI agents at GW took . . . . There was a confrontation on the scene with the military adviser or aide," Ruff said, adding, "The matter was treated as a separate matter apart from the assassination investigation, and it was resolved to my best knowledge within 36 to 48 hours."

Evidence taken by the FBI that day included bags of hospital trash and bed linens, as well as the president's personal code card. The trash was later returned to hospital officials. But even after a comprehensive investigation Reagan's tattered shirt and one gold cuff link, an item of sentimental value to the president, have never been found.