An abortive coup yesterday and the departure of two of its leaders into exile raised questions in government and military circles today about Thailand's lenient attitude toward coup plotters.

Military and civilian officials confirmed that Manoon Roopkachorn, a former Army colonel dismissed for his role in a 1981 coup attempt, and his younger brother Manas, a serving Air Force wing commander, left Thailand last night with two aides only hours after about 400 troops loyal to them abandoned a bid to seize power and surrendered to government forces.

The latest attempt to overthrow the government of Prime Minister Prem Tinsulanond left at least five persons dead, including two foreign correspondents, and 59 injured. NBC bureau chief Neil Davis, a 52-year-old Australian, and American soundman William Latch, 35, were killed by rebel tank fire in downtown Bangkok. The other fatalities included a Thai woman passenger in a taxi and two government soldiers.

Among the wounded were several civilian passengers of a bus hit by shrapnel from a rebel tank round. At least 22 soldiers were reported wounded in the fighting.

As Bangkok returned to normal today, Prem said that an investigative commission was being formed to look into the coup attempt and determine the plotters and participants. But he had no comment on the departures of Manoon and Manas, and he appeared to defend a surrender deal that ended the coup attempt by allowing the two instigators to leave the country.

"The government has avoided using drastic measures in order to maintain our unity," Prem told reporters today. He returned to Thailand yesterday after cutting short a trip to Indonesia.

In contrast to Prem's low-key response, an editorial in the English-language newspaper The Nation today harshly denounced the coup attempt as "a plot hatched by an infertile brain and carried out with maximum inefficiency."

It condemned the "handful of ex-military persons suffering from delusions of power" who organized the takeover bid. Despite this "ridiculous power play," The Nation said, "democracy is here to stay, and there is no room for those who hallucinate."

Government sources also said the coup attempt and its aftermath had aroused considerable anger, and there were indications that some military men shared civilians' objections to the leniency shown the plotters.

"There's going to be questioning of the traditional method of the military on what to do with plotters," said a government official who did not want to be identified. He indicated that some authorities were disturbed that the "Thai way of forgive and forget" would be practiced this time, as it was in 1981, and were demanding "a real accounting" in view of the casualties and damage.

"It sets Thailand back quite a bit," the official said. He expressed concern that the "political and economic repercussions" of the coup attempt would cause further damage at home and hurt the country's image abroad.

Deputy Army commander Gen. Thienchai Sirisamphan said yesterday, while the coup attempt was in progress, that its leaders faced execution once it was quelled.

Today he acknowledged that Manoon and Manas had boarded a plane for Singapore at Bangkok Airport, where they were seen off by senior military officers. The brothers reportedly were allowed to take a light Air Force passenger plane on a special flight arranged by senior loyalist officers in return for releasing the Thai Air Force commander, abducted during the coup attempt, and averting a major battle with government forces in downtown Bangkok.

Government sources said Singapore was only a "transit" destination. There was speculation that Manoon would try to fly on to the United States, where he has spent considerable time since the 1981 coup attempt.

In Washington, the State Department confirmed that Manoon and two of his aides had applied for U.S. visas. Department spokesman Bernard Kalb said the applications would be reviewed "in accordance with the relevant data." Consular officials said special attention would be given to the applications because of the "sensitive" nature of the case. A decision is expected within the next few days, they said.

One government official said Manas had fled to Burma, but no other details were available. Singapore has refused to comment.

In the absence of Manoon and Manas, the main task of the investigative commission will be to determine the role of three former generals implicated in the coup attempt, officials said.