The accidental U.S. missile attack the past weekend on an Iranian airliner, killing all 290 people on board, was the deadliest recorded military downing of a civilian plane.

Consequently, America has plenty of explaining as well as plenty of soul-searching to do. But there are sharp limits to the recriminations the U.S. should heap upon itself or accept from others.Meanwhile, the most constructive thing that can be done immediately is not to pinpoint blame but to find the answers to some disturbing questions.

For openers, why did the commander and crew of the USS Vincennes, considered the most sophisticated combat ship in the world, mistakenly conclude that the Iranian plane was a fighter jet rather than a civilian airliner? Did the ship's radar misfunction? Or was the radar misinterpreted?

How far should the U.S. go in trying to repair the damage? Does this tragedy discredit America's continued military presence in the Persian Gulf? Should the Washington bow to the increased pressure this episode has generated for the U.S. to pull its ships out of the gulf?

While some of these questions cannot be answered until an investigation is completed in about two weeks, a few key points should be clear already:

First, as long as the U.S. remains in the Persian Gulf, it is going to be in a no-win situation. That is, the U.S. is going to be criticized if its ships respond too soon to a perceived threat - and criticized if they respond tardily or not at all.

Second, though Americans pulled the trigger, Iran bears by far most of the blame for this appalling loss of life. As long as Iran repeatedly resists efforts to call off the war with Iraq and then sends a commercial airliner into a combat zone, accidents like the one the past weekend are bound to happen.

Third, while the blame is mostly Iran's, Washington still should offer to make some reparations to give substance to President Reagan's expressions of regret and to back up the point that, unlike many other nations, America does not intentionally make war on innocent civilians.

Fourth, since the extremist leaders of Iran seldom listen to reason, the U.S. had better brace itself for a new round of terroristic attacks on Americans not just in the Middle East but wherever else Iran can reach.

Fifth, as part of its investigation of this episode, the Navy should take a particularly close look at the Vincennes' system for identifying whether or not approaching aircraft are hostile. Long before the current tragedy, American admirals were deeply divided on the system's capability.

Sixth, what about the U.S. withdrawing from the Persian Gulf to prevent further such episodes, including those that could involve the substantial loss of American lives? No, a withdrawal still would be a mistake. While the U.S. could get along without oil from the Persian Gulf, some of our allies can't. Besides, the oil lanes of the gulf comprise an international waterway, and there is an international obligation to keep them open. Moreover, a U.S. presence is needed to keep Russia from turning the Persian Gulf into a Soviet lake.

Finally, this tragedy should at least breathe new efforts into Washington's efforts to get the United Nations to impose an arms embargo on Iran and Iraq. As those efforts proceed, let it not be forgotten that Russia and China, which have been the quickest to condemn the U.S. for the downing of the Iranian airliner, have repeatedly thwarted the proposed U.N. embargo. Consequently, Russia and China bear some responsibility for the new tragedy.