CLINTON AUGMENTS AIRPORT SECURITY : TWA FLIGHT RECORDER CAPTURES UNKNOWN SOUND.

Byline: James Bennet The New York Times

After spending almost three somber hours with relatives of the victims killed in the crash of Trans World Airlines Flight 800, President Clinton announced Thursday that he had ordered several steps to tighten security at the nation's airports, including heightened scrutiny of passengers, luggage and cargo.

Clinton said the new security measures were intended to calm the nerves of travelers worried by the crash, but he added that the government had not yet drawn any conclusions about what caused it.

``While we seek the cause of the disaster, let us all agree that we must not wait to alleviate the concerns of the American people about air safety and air security,'' Clinton said in remarks to reporters at John F. Kennedy International Airport.

Also Thursday, investigators examining the plane's cockpit voice recorder, pulled from the wreckage in the waters off Long Island early Thursday morning, said the device captured a sound lasting just a fraction of a second before it stopped functioning. While no one was willing to draw firm conclusions from that, law enforcement officials said it supported the theory that the plane was destroyed by a bomb. At the crash site, searchers recovered the remains of 12 more people Thursday, leaving 103 bodies still to be pulled from the water.

Clinton said the new security measures, to take effect immediately, could increase the cost and inconvenience of air travel, but added, ``The safety and security of the American people must be our top priority.''

Emphasizing that he did not know what caused the crash of Flight 800, he said: ``If it proves to be a mechanical failure, additional safety measures may be required. If it proves to be a criminal act, other security steps may be required. Whatever needs to be done, we will do it.''

Referring to the crash, an airport security directive issued by the Federal Aviation Administration on Thursday said, ``A determination as to the cause of the incident has not been made and it is difficult to ascertain when that determination will be made, but a terrorist attack by means of an improvised explosive device cannot be ruled out.''

Clinton also said he had asked Vice President Al Gore to lead a commission to review aviation safety and airport security, as well as the pace of modernization of the air traffic control system. Gore is to report back to the president within 45 days. Clinton said Gore would come up with a plan to deploy new machines that can detect sophisticated explosives in baggage, and perhaps cargo as well.

After days of mounting anger over the pace of the investigation, family members and even some of Clinton's sharper critics seemed heartened by the visit.

``He's gone a long way to reassuring people that we are concerned, that the country is concerned and that all is being done that can be done,'' said Sen. Alfonse D'Amato, R-N.Y., who accompanied Clinton on Thursday.

Administration officials characterized the new safety measures as the third in a series of government steps to make flying safer. They said airport officials immediately would begin hand-inspecting more bags and interviewing more passengers. The directive said the searches would be frequent enough ``so that a physical search is ongoing at each X-ray unit at all times.''

David Hinson, the administrator of the Federal Aviation Administration, said that because of the new measures, travelers should expect delays ``from 15 minutes to a little over a half hour,'' at least at first. He added that the delays should ease as airlines master the new rules.

Officials urged passengers to arrive at airports earlier, with photo identification ready and luggage clearly labeled.

Echoing the president, administration officials repeatedly said that the issuance of the new security measures Thursday did not mean that conclusions had been drawn about what caused the crash of Flight 800.

``The security measures that are being put in place are simply prudent and responsible measures to institute in light of a longstanding security threat in our country,'' said Federico Pena, the secretary of transportation. He also accompanied Clinton.

But at a separate crash-investigation briefing Thursday afternoon, James Kallstrom, the FBI agent in charge of the inquiry, said: ``We know there was a catastrophic explosion; it was caused by some kind of bomb, obviously explosion.''

But then Kallstrom quickly added, ``We're not further describing how that would be, whether it was carried on, whether something hit the plane from the outside and caused it to explode, or a mechanical problem that has nothing to do with terrorism or criminality; or something in the cargo that would have caused a mechanical problem.''

Since the crash, investigators have said they believe the jet could have been destroyed by a bomb, a surface-to-air missile, or an unknown, catastrophic mechanical problem.

Federal and airline industry officials said the government was considering imposing similar security measures before Flight 800 crashed July 17. After the crash - almost as soon as the possibility of sabotage was raised - the White House ordered officials to prepare to announce the new steps.

But as the investigation dragged on, the administration was placed in a bind. By announcing the measures, it might appear to be prejudging the investigation's outcome. The administration also risked looking intemperate if the crash proved accidental. On the other hand, by saying nothing, the White House ran the political risk of seeming uninterested while the nation heard repeated talk about the possibility that the plane was destroyed in a terrorist attack.

Officials said that Thursday was the earliest day Clinton could visit the families because he spent the early part of the week campaigning in the West.

Administration officials had worried that the public might infer from Thursday's announcement that the administration believed terrorists, rather than a mechanical failure, had downed the TWA jet.

``That was a consideration,'' said George Stephanopoulos, the president's senior adviser. But, he added, ``We felt it prudent to reassure people, even before a final judgment was made, about the safety of the airlines.''

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Photo

PHOTO Bernard Loeb of the National Transportation Safety Bo ard displays the flight data recorder and cockpit voice recorder.

Associated Press