SEOUL, DEC. 1 -- An Asian couple linked to the disappearance of a Korean Air jet swallowed poison capsules during questioning in Bahrain today and South Korean officials said they suspect the two had planted a time bomb that caused the plane carrying 115 people to explode in the air Sunday.

As a bizarre tale of forged passports, false identities and international intrigue unfolded here, in Tokyo and in the Middle East, officials said they are convinced that sabotage caused the plane to crash.

Suspicion focused on the couple, two passengers holding Japanese passports who flew on the plane from its origin in Baghdad and then got off during a refueling stop in Abu Dhabi.

The couple, a man and a woman, were the only passengers to leave the KAL flight when it stopped in Abu Dhabi. The couple later flew to Bahrain.

While they were waiting to be questioned by Bahraini immigration officials, the two took poison capsules hidden in cigarettes, correspondent Patrick E. Tyler reported from Manama. The man died and the woman was hospitalized in serious condition. She is expected to survive.

KAL Flight 858 disappeared Sunday while flying over Burma toward a refueling stop in Bangkok. Villagers on the Thai-Burmese border reportedly heard an explosion, and airline officials here said they believe all 95 passengers and 20 crew were killed.

{Search teams have sighted the wreckage of the jet in Burmese territory controlled by antigovernment rebels, United Press International reported from Kanchanaburi, Thailand. A Thai police officer said the site of the crash was on the Burmese side of the Salween River separating Thailand and Burma, opposite the Thai village of Bong Thi.}

The KAL pilot of the four-engine Boeing 707, after radioing his position to air traffic controllers in Rangoon, sent no distress call.

"We have considered many possible causes, but by all indications, bomb explosion is the most probable cause of the incident," a KAL spokesman here said. "The plane was equipped with four engines and advanced communications systems, and any technical troubles should have been signaled. Only a sudden terrorist attack such as a bomb could have caused the crash."

Immigration officials in Manama were alerted to watch for the Asian couple by Korean and Japanese diplomats, and stopped the two from boarding a Rome-bound flight today after noticing that they carried only hand luggage. Officials discovered that the woman's passport was a forgery, and detained them for questioning.

"They took poison capsules out of the cigarettes and swallowed them," said Takao Natsume, Japan's senior diplomat in Manama. "Within seconds, they fell down and their bodies became stiff."

"I very strongly feel that they may have planted the bomb," said South Korean Assistant Foreign Minister Park Soo Gil. "By all evidence available, that seems to be the case."

Officials in Seoul said they believe the two were affiliated with a North Korean support group in Japan. Foreign Minister Choi Kwang Soo later told reporters that he strongly suspected ties to a North Korean residents' association in Japan.

But officials said it is also possible that the Japanese Red Army, a small terrorist group with members now living in North Korea and the Middle East, was involved.

Many details about the possible involvement of the two remained hazy or conflicting; even their nationality was unsure tonight. But officials in the foreign ministries here and in Tokyo offered this outline:

The two were traveling as father and daughter with passports in the names of Shinichi and Mayumi Hachiya. Her passport was clearly fake, officials said.

Shinichi's passport seemed genuine, however, and a Shinichi Hachiya is a Tokyo resident. Tonight, Hachiya, 69, was interviewed in Tokyo and offered an explanation of how someone might have obtained a passport in his name.

Hachiya said that two or three years ago, when he was planning a trip to Hong Kong, a business acquaintance named Miyamoto offered to make arrangements for him and held his identity papers for about a month. The real Hachiya said that Miyamoto was an ethnic Korean living in Japan.

{Yomiuri Shimbun, quoting police sources, reported in its Wednesday editions that an Akira Miyamoto was linked to a North Korean spy case in March 1985 and is wanted by Japanese police.}

Japanese officials said that someone using Hachiya's name bought a Korean Air ticket in Tokyo on Nov. 19. They did not say which KAL flight ticket was purchased.

South Korean officials now believe that the pair traveled to Vienna around Nov. 19 and from there to Belgrade on Nov. 23. They flew from Belgrade to Baghdad on Nov. 28, and then boarded KAL Flight 858 in the Iraqi capital. After leaving the plane in Abu Dhabi, the two spent 12 hours on the ground and then flew Gulf Air to Bahrain, according to officials in Tokyo.

KAL crew members who left the plane in Abu Dhabi said the two acted suspiciously during the flight. Unlike most Japanese, for example, the two did not say thank you when served food.

A South Korean Embassy official in Bahrain became suspicious when he noticed they came from Yugoslavia, and visited the two in their hotel. The Korean charge d'affaires reported that the two seemed to speak Japanese and acted nervous. About two hours later, the two went to the airport and tried to board a plane for Rome, via Amman.

Officials in Seoul are studying whether they can request the woman's extradition from Bahrain.

Officials here have warned that North Korea might commit sabotage to interfere with South Korea's presidential election, scheduled for Dec. 16, or to disrupt planning for the Olympics, scheduled to be held here next September.

Separately, Japanese police recently arrested a longtime fugitive member of the Red Army. He was arrested in Japan with a ticket to South Korea in his possession. Police said they also found a letter from the Red Army attacking the Olympics as a tool of U.S.-Japanese-South Korean imperialism.

The Red Army, which is believed to have only about 30 members, was responsible for several acts of terror in the 1970s, including hijackings and a massacre at Lod airport in Israel. It has been dormant for several years, however. Correspondent Margaret Shapiro and special correspondent Shigehiko Togo in Tokyo contributed to this report.