Two airmen, their sensing equipment indicating dangerous levels of toxic fuel, had just left a passageway adjacent to a Titan II missile silo near here when a spectacular explosion reduced the silo to rubble, an Air Force spokesman said today.

One of the airmen, Sgt. David Livingston of Columbus, Ohio, was killed, and the other, Sgt. Jeff Kennedy of Portland, Maine, is in critical condition in a medical center in Little Rock. Twenty others were injured in the explosion.

In a news conference, Lt. Gen. Lloyd R. Leavitt Jr., vice commander of the Strategic Air Command in Omaha, said Livingston and Kennedy were the second trouble-shooting team sent into the silo in the hour before the explosion to monitor pressure building dangerously inside.

"We decided to send a crew in to monitor the reading so we could figure out a way to vent the pressure [buildup] and prevent an explosion," Leavitt said. "We knew there was considerable risk to the people concerned, buy they understood the risk and we felt it was our obligation" to proceed.

In response to a question about whether the troubleshooters could have caused the explosion by perhaps letting air into the silo that could serve as an oxidizing agent with the fuel, or triggered the explosion in some other way, Leavitt said. "To the best of my knowledge, they did nothing to cause it." He termed them "victims of unfortunate bad luck."

Livingston and Kennedy had just reached the lip of an access area above ground 75 feet from the silo when the explosion occurred with force enough to blow off the missile's 740-ton door of reinforced concrete and steel, designed to withstand a near-miss of a 10-megaton enemy nuclear warhead.

Livingston died of multiple injuries, including pulmonary complications which resulted from his inhalation of toxic chemicals, a hospital spokesman said.

Leavitt said the two airmen were under efforts to evacuate the area the moment their sensing equipment indicated dangerous levels of toxic fuel underground.

The general said an accident investigation board was gathering information in an attempt to determine the cause of the explosion.

"We know there was a buildup of pressure [from leaking fuel mixing with the missile's oxidizer], perhaps to an explosive level inside the silo," he said. He also speculated that the missile's fuel tanks, at the bottom of the silo, could have collapsed, causing the missile to crumple and the fuel to combine with its oxidizer and explode. He said the "critical" question was, "did the fuel-air mixture reach a combustible point? The accident board will get into that."

Livingston and Kennedy entered an access area between the silo and the crew capsule underground to test for toxic gas after another two-man crew ran out of oxygen, according to Leavitt. The first crew had trouble trying to open the first of two sealed doors that separate the missile's underground crew-command post from the silo.

The Air Force dispatched trouble-shooters because the silo crew had evacuated earlier and could no longer monitor the levels of toxic fuel inside the silo.

Livingston and Kennedy encountered "poor visibility"; and their meters registered the "upper limits of toxicity" once they entered a passageway leading to the silo, said Leavitt. He said the level of fuel contamination was so high, it "wasn't safe for them even with their suits on . . . we don't know how high it was because the device they carried is calibrated well below the explosion point."

Meanwhile, Air Force officials decided against a second evacuation today of the homes around the missile site.

The second evacuation had been considered in a meeting with local and state officials Friday, presumably to allow military authorities to retrieve the missile's jettisoned nuclear warhead, local sources said today.

It was unclear today whether the nuclear warhead, reportedly flung some 200 yards from the silo, had been moved from the site, or even had been found. Air Force officials would "neither confirm nor deny" the existence of a warhead.

A government source, however, said the warhead had been found, and the Air Force is planning to truck it from the destroyed Damascus site "as soon as possible." It will be taken to the Little Rock Air Force Base and then to another site for study.

[County authorities said the Air Force told them tonight "the area was secure" -- which they interpreted as a sign the warhead had been recovered, United Press International reported.]

After being catapulted out of the silo, the warhead landed "on government property" that extends beyond the immediate area of the missile site, the source said. The missile, he added, "just disintegrated."

In an arrangement worked out between the Air Force and Arkansas state officials the Air Force has promised to notify state law enforcement authorities if they plan to move any nuclear weapons so the state can provide a convoy. Bob Lyford, Gov. Bill Clinton's aide for emergency services, said no such request for police escort had come into the governor's office.

Outside the site in rolling Ozark foothills, about 50 Air Force personnel were clearing the debris around the silo.

Capt. Don Schaefer, ASAC information officer, said the Air Force was continuing to monitor the site with Geiger counters and vapor detectors to check for any radioactivity or toxic fuel traces that might remain.

"There is no radioactive material in the vicinity," he said. "We want to allay fears . . ." He declined to comment on whether an undetonated warhead could be considered radioactive material.

Local officials remained bitter toward the Air Force over the lack of information provided them during the crisis that required the evacuation of about 1,400 people in the tiny towns of Damascus, Bee Branch and Gravesville early Friday morning. Residents were allowed to return home this morning.

"From the beginning to the end of this thing there has been no contact between the Air Force and us," grumbled Van Buren County Sheriff Gus Anglin, 44, who was at the site when the silo erupted.

"With the Air Force you don't know who's in charge. You get someone and they have to confer with someone else. I've even tried to contact SAC in Omaha but I got no satisfaction so I just stopped trying."