ONE day after unknown assailants released nerve gas on five train coaches in the Tokyo subway system, Japanese celebrated the spring equinox, 24 hours of perfect balance, when night is only as long as day.
For the most part, people marked the holiday by staying home. Some visited cemeteries to pay respects to their ancestors. Tokyo seemed quiet and reflective, as if the city had regained a solemn equilibrium.
Police, according to local news reports, announced they would question a hospitalized man who witnesses say left a newspaper-wrapped parcel containing the gas on one train, but are waiting for his recovery. Service returned to normal on subway lines disrupted by the attack, which left eight people dead and injured almost 5,000.
But behind the return to stability was a profound sense of unease. In addition to predictable expressions of concern about Japan becoming a more dangerous place, the event's odd characteristics have left deeper worries among Japanese.
Many people are puzzled by both the novelty of the method -- some experts have said the incident may be the world's first use of chemical weapons by urban terrorists -- and the silence of the perpetrators. These factors suggest something different from ordinary terrorism is at work.
The event comes at a time of turmoil in Japan -- the country's political establishment is in a long period of realignment, the economy is in a protracted stall, and some Japanese complain about a lack of national purpose. Many people, in and out of Japan, criticize the government for regulating the society tightly, but there is also an unspoken feeling of comfort with the society that government has created.
''I don't think the criminals targeted the bureaucracy,'' says a young man at one of the subway stations where the gas struck. Declining to give his name, he argued that one could have targeted government institutions more directly. ''The group has some dissatisfaction against society as a whole,'' he says. ''They wanted to release their frustration against this society.''
The mystery has led some people to raise the possibility that one of Japan's small religious groups may be involved. A sect called Aum Shinri Kyo, or Aum Supreme Truth, has been linked to a previous appearance of the kind of gas used in Tokyo. But the group has twice denied any connection to the attack.
This year is the 50th anniversary of the end of World War II, and the government's discussion of a resolution apologizing for Japan's role in the war has prompted opposition from conservatives. At the same time, veterans and researchers have come forward with new disclosures about Japan's wartime atrocities, including the use of nerve gas in experiments in China. No one is pointing fingers at the rightists, but some suspect a connection between the March 20 attack and the war.
Other observers have noted that the three subway lines targeted by the assailants all stopped at a station called Kasumigaseki, the bureaucratic quarter of the Japanese capital and the locus of power in a country guided by public servants.
They speculate that the attack was a lash at the power structure.
''At many times we have had challenges to state power from the right and left,'' says Shiichi Yoshida, a company worker interviewed as he got off the subway at the station. But, he adds, there is no way to determine who might have orchestrated this challenge.
Political expression in Japan tends to be noisy. Right-wing protesters favor sound trucks that blast their message and martial music in public places. Leftists here are famed for firing rockets at official targets. Neither side has been reluctant to claim credit for its activities. Nerve gas is noiseless, just as quiet as the unidentified group that police allege is responsible for the March 20 incident.
''I have no way of understanding why this could have happened,'' says another subway rider, identifying himself only as a businessman. He says he was disappointed that police had not solved a June 1994 case in which seven people died from the same gas, called sarin, in the city of Matsumoto. ''If they had done a better job in Matsumoto, such a thing could never have happened,'' the man says of the attack.
At the same time, the man was concerned about the reaction of the police. '' As a result,'' of the Tokyo incident, he says, ''there may be a strong crackdown that would be ... dangerous.''
Motoko Shiraishi, a university student, says the event had changed her outlook. ''Now that I know that there is such a group, I'll be careful about my safety.'' She voiced a concern that often arises here after incidents of societal dysfunction: ''Tokyo is becoming closer to New York.''