The sources include officials from a variety of U.S. agencies involved in gathering and analyzing intelligence about Cuba and Central America. All spoke on the condition that neither they nor their agencies be identified.
An administration official who monitors Cuban and Nicaraguan affairs said the United States believed that the MiGs flown by the Sandinista training squadron were originally intended for Nicaragua but never got there because of U.S. threats of reprisal.
He said the United States suspected that the jets were shipped to Cuba two years ago and were later incorporated into the Cuban air force when Managua decided against risking U.S. retaliation.
Although they say they do not have confirmation, U.S. intelligence analysts say they are virtually certain that Sandinista pilots are flying regularly
from San Julian, Cuba's second-largest air base, about 100 miles southwest of Havana. San Julian is about 600 miles from Punta Huete, Nicaragua's largest military air base, which is already configured to service MiGs.
"I have no information on whether the Nicaraguans have operational control (of MiGs in Cuba), but I can tell you that Nicaraguans have received MiG-21 training in Cuba," said a Defense Department spokeswoman, Capt. Nancy LaLuntas.
The Nicaraguan Embassy and the Cuban Interests Section in Washington declined to comment. But sources close to both governments acknowledged that it had been known for some time that Cuba had trained Nicaraguans.
What appears to be new is the identification of the training base and the indication that Nicaraguan pilots have formed a unit that has informal possession of some Cuban MiGs.
Between 10 and 17 MiG-21s at San Julian are believed to be assigned to the up to 40 Nicaraguan pilots training there at any given time. Sandinista mechanic trainees are also rotated at the base regularly.
The full complement of Nicaraguan pilots and mechanics is estimated to be about 60. A Cuban-style MiG-21 squadron consists of 10 to 15 aircraft; in combat the MiGs patrol or attack in pairs.
San Julian, built by the United States during World War II to train B-29 bomber crews, houses Cuba's main military pilot training school. It is believed to be under the jurisdiction of the Western Air Brigade, one of three Cuban national air wings. Brig. Gen. Rafael del Pino Diaz, the Cuban officer who defected to the United States May 28, reportedly was the Western Brigade's chief air combat instructor.
U.S. officials said that del Pino or his subordinates may have supervised the Nicaraguans, and that they intend to ask him about it during his debriefing sessions.
The officials said Cuban instructors retain ultimate control of the MiGs, and the Sandinistas would not be able to unilaterally order their pilots on missions from Cuba.
However, they said they understood that the Sandinista pilots are experienced fliers - and added that the Sandinista crews are probably rehearsing combat responses to an American invasion, a naval blockade or limited tactical air strikes against Nicaragua.
Moreover, the sources said, it is possible the Sandinista pilots already have specific targets in their combat briefing books. From San Julian, a MiG- 21 could strike as far north as Savannah, Ga., and as far south as San Jose, Costa Rica.
The MiG-21 can fly as fast as 1,285 m.p.h. and has a range of 683 miles, growing to 1,118 miles with disposable external fuel tanks. A fully armed MiG- 21 launched from San Julian could carry out combat missions in Nicaragua but could not return because it would run out of fuel. However, the sources said the MiGs could be rearmed and refueled at the Punta Huete air base.
The sources said they concluded that the Nicaraguans were flying out of San Julian from statements by Nicaraguan defectors and analysis of pictures taken by spy aircraft and satellites controlled by the National Reconnaissance Office, a secret unit that oversees overhead photo intelligence.
Retired Army Col. Lawrence Tracy, who helped prepare a 1984 study of the issue, said Bulgaria trained the first Sandinista MiG pilots between late 1980 and late 1982. He and others said the Nicaraguans then were reassigned to Cuba to maintain their flying proficiency while awaiting the formal arrival of MiGs in Nicaragua.
That has not happened because of systematic U.S. warnings that the acquisition of MiGs by the Sandinistas would provoke a retaliatory U.S. response.
In February 1984, when U.S. intelligence warned of the impending delivery of MiGs to Nicaragua, President Reagan signed a secret national security directive instructing that Moscow and Havana be warned that the United States ''will not tolerate the introduction into Nicaragua of advanced fighter aircraft or Cuban ground forces," according to a copy of the directive obtained by The Inquirer Washington Bureau.
Nine months later, Moscow assured Washington that it would not provide Nicaragua with sophisticated aircraft, according to U.S. sources. But they said that Havana was silent, possibly because the San Julian arrangement had been reached.