President Reagan says the Navy was justified in destroying an Iranian jetliner with 290 people aboard because U.S. officers thought it was an attacking warplane. Iran called the action an "act of terrorism" and promised retaliation.The president promised a full investigation into still-unanswered questions about the Persian Gulf tragedy. Rear Adm. William M. Fogarty started for the Middle East Monday morning to piece together the facts, but details of his itinerary were not available, said Lt. Cmdr. Chris Baumann, a Pentagon spokesman.

Reagan, who returned to the White House Monday to wrap up his July 4th holiday, issued a statement Sunday afternoon saying "our sympathy and condolences go out to the passengers, crew and their families."

But he strongly defended the missile attack early Sunday morning by the cruiser USS Vincennes, saying the approaching jetliner ignored warnings and the American ship "followed standing orders and widely publicized procedures, firing to protect itself against possible attack."

The incident immediately provoked comparisons to the Soviet destruction of a Korean Air Lines jetliner in 1983, an attack that killed 269 people and led Reagan at the time to de-nounce "the barbarity of the Soviet government in shooting down an unarmed plane" after it veered into Soviet airspace.

Adm. William Crowe, the chairman of the Joint Chiefs of Staff, rejected such comparisons, saying the Iranian airliner was flying in a "combat zone" where Iran itself wages war.

But numerous questions remain about how the mistake occurred, including why the Navy's most sophisticated naval defense system did not distinguish the large Airbus from Iran's much smaller, U.S.-made F-14 attack fighters.

Crowe said the Aegis defense system "cannot defy the laws of physics" and that "one of the most difficult problems is from a radar blip, particularly from a head-on target, to identify the type of aircraft." He also said the Vincennes' crew had only about four minutes to make a decision after picking up the approaching aircraft.

Members of Congress are scheduled to end their July 4th holiday on Wednesday, with the incident certain to renew debate over the Reagan administration's policy of military deployment in the gulf.

House Majority Leader Tom Foley, interviewed on NBC-TV's "Today" program Monday, said all information about the destruction of the jet should be released publicly "to demonstrate that the United States does not make war on innocents."

But the Washington Democrat said "it would be a great mistake for the United States to abruptly withdraw its forces."

Sen. Dale Bumpers, D-Ark., said the U.S. policy in the Persian Gulf "was never a very good one to begin with, and this incident, this sort of thing was very predictable."

Sen. Richard Lugar, R-Ind., interviewed with Bumpers on CBS's "This Morning" program, said "the policy of trying to protect commercial shipping in the Gulf is absolutely right."

Democratic presidential nominee-to-be Michael Dukakis said, "Plainly our armed forces have a right to defend themselves when attacked" and urged Iran to reconsider its threats to respond with terrorism. He said, however, the downing of the plane is further evidence that "the war between Iran and Iraq must end."

Dukakis' Republican counterpart, Vice President George Bush, broke his silence Monday by expressing regrets about the loss of life but defending the ship's captain, saying "he did what he had to do."

In Chicago, civil rights leader Jesse Jackson, was more critical of the administration and called Sunday's incident "regrettable."

"It is another result of our being in the gulf for more than a year without a definite policy." Jackson also called for an end to the 8-year-old war.