The bomb blew off the hand of a secretary who was opening the package. No one took responsibility, and it was unclear if it was linked to the crackdown on the sect. But it undermined the initial relief felt after the arrest of sect leaders on murder and other charges arising from the nerve gas attack on the Tokyo subway system in March.
Because of the fears that sect members may stage retaliatory strikes, the army's chemical warfare units and 80,000 police officers were put on alert on Tuesday around Japan. Security was especially tight in train stations and other public places.
The police on Tuesday arrested 15 of the 41 sect members for whom arrest warrants were issued Monday night in connection with the nerve gas attack, which killed 12 people and injured 5,500. Twelve sect members were already in custody, so 14 still remain at large.
Prime Minister Tomiichi Murayama, speaking in a nationwide address, warned that some sect members may still possess sarin nerve gas.
"We will make the utmost efforts to prevent a recurrence of sarin attacks, mainly through thorough police work," Murayama said.
The sect's guru, Shoko Asahara, who is suspected of masterminding the nerve gas attack on the subway system, was arrested for murder after he was discovered meditating in a secret chamber in the sect's headquarters, near Mount Fuji. The police had to use crowbars to break into the hiding place, which was three feet high and the size of a very large coffin.
Asahara and his disciples now may face the death penalty, by hanging. Those convicted of multiple murders are often hanged in Japan.
Japanese television, drawing from a police briefing that foreign journalists were not allowed to attend, said that in his first interrogation Asahara had denied any involvement in the subway attack.
"How could I, as a blind man, have possibly done such a thing?" Asahara was quoted as saying. Asahara, 40, has very poor vision, although he can see enough to walk around.
Although the sect has repeatedly said that Asahara is gravely sick, he was able to walk without difficulty to the van that sped him to Tokyo.
The public responded to the arrest with widespread elation. People snapped up free extras that newspapers handed out in subway stations, and crowds turned out to watch along the highway as a convoy of police cars with lights flashing delivered Asahara to the Tokyo police headquarters.
The police have been concerned all along that the arrest of Asahara would set off retaliatory terrorism. A top police official said he doubted that the sect still had sarin, but other government officials said that it might.
One reason for the police's view may be statements made under interrogation by the sect's top chemist, Masami Tsuchiya. The Asahi Shimbun, a Japanese daily, reported that Tsuchiya told the police that the sect had disposed of its sarin stockpile as a way of destroying evidence.
Another newspaper, the Yomiuri, said Tsuchiya had told the police that in any case the sarin he made deteriorated quickly. Normally, pure sarin keeps for many years if stored in an airtight container, but there are some indications that the sect's sarin was impure or contained too much isopropyl alcohol.
Impurities may be one reason why the subway attack killed only 12. More than three quarts of sarin were used, diluted with another three quarts of solvents, but such a quantity of sarin normally would have killed far more people.
The parcel bomb that exploded on Tuesday night was addressed to Tokyo's governor, Yukio Aoshima, an elected official whose position is equivalent to that of mayor. Aoshima was out of the building when his secretary opened the book-sized package.
"There suddenly was a big blast," a cleaning woman who had been in the room told reporters. "The man had lost his fingers, and he was shivering in shock, and everybody ran around in confusion."
The secretary, Masaaki Usumi, lost his left hand and right thumb in the explosion.
Television news programs were reporting the elation over Asahara's arrest when they had to interrupt the programming to bring reports of the bomb blast. Aoshima hurried to the scene and said he was shocked by the incident.
"I'm going to make sure this never happens again," he declared.
After the arrests of Asahara and his aides, the government began proceedings to deprive Aum Shinrikyo of its status as a religious corporation. This would mean that it could not own property in its own name and would not enjoy tax benefits, but it could still continue to function as a religious organization.
The sect had nearly 10,000 believers in Japan, and more than 20,000 abroad, mostly in Russia, and by all accounts ordinary believers knew nothing about alleged criminal activities by the sect. Many were attracted by its yoga classes, its borrowings from Buddhist teachings, and its emphasis on scientific experimentation.
Home Affairs Minister Hiromu Nonaka appealed to these sect members not to seek revenge against the public.
"The case is not over, because some Aum members may still have sarin and weapons," Nonaka said. "Aum members should consider themselves Japanese nationals, part of a democratic society, and should act responsibly."
One of the few top officials of the sect who was not arrested was Fumihiro Joyu, the sect's spokesman, who has become a major television personality and a heart-throb to some Japanese high school girls and young women -- scandalizing their parents. In a press conference, Joyu denounced the arrest of Asahara as religious repression, and he said Aum Shinrikyo would continue to operate.
"This was an unjustified arrest," Joyu said. "But followers took it calmly, like Buddhists."
The authorities also removed 44 children from a sect center on Tuesday to see if they were being mistreated. The youngsters apparently are the children of Aum believers, but there have been widespread reports that children in sect centers have little contact with their parents and are subjected to sleep deprivation and rigorous controls.
Police and prosecutors already have a good deal of evidence, partly because the Japanese custom is for the police to arrest someone only when they have just about enough evidence to win a conviction in court. The police are very proud that, as a result, nearly everyone arrested is indicted, and 98 percent of those indicted are then convicted.
The trial is public but there are no juries. Instead, Japan uses the same system as the European mainland, of a panel of three judges in serious cases. The judges listen to witnesses and then determine the question of guilt as well as the appropriate sentence.
The next step is for Asahara and his aides to be interrogated in the police station, for a period that police can extend for up to three weeks. During this time, anonymous police interrogators grill suspects all day and typically put great pressure on suspects to confess.
There are widespread reports that police sometimes rough up suspects or deprive them of sleep to help induce a confession, and in this period suspects have only limited access to a lawyer.
The police case against Asahara will apparently rely partly on confessions of several of his aides, and also on strong circumstantial evidence. For example, the chemicals found in the subway are identical to some found in the sect's chemical laboratories, and a sealing device in the headquarters leaves precisely the same kind of imprint as found on the sarin bags left on the subway.
Among the questions that the authorities are likely to put to Mr. Asahara are why the sect felt a need to build the chemical laboratories where sarin allegedly was produced, plus an industrial-sized chemical plant that may not have been operational but that in theory could have produced a ton of sarin at a time.
The authorities and public are also interested in learning about the motivation for the sect's reported machine-gun factory, bacteriological warfare research, and interest in buying Russian tanks and nuclear warheads.