Tokyo (CNN) Seven members of the Aum Shinrikyo cult, which carried out the deadly 1995 sarin gas attack on the Tokyo subway, have been executed, Japanese officials said Friday.

Cult leader Shoko Asahara, whose real name is Chizuo Matsumoto, had been in prison for 22 years before his execution this week. The attack left more than a dozen people dead and thousands injured.

Twelve other members of Aum Shinrikyo were sentenced to death for their roles in the Tokyo attack. Asahara's death sentence was finalized in 2006, according to public broadcaster NHK, but trials of his co-conspirators dragged on for a further 12 years.

Since those proceedings finished earlier this year, the days of the Aum Shinrikyo members had been numbered, even as opponents of the death penalty attempted to block the executions.

Asahara was one of seven members of the cult hanged this week. The others are Tomomasa Nakagawa, Tomomitsu Niimi, Kiyohide Hayakawa, Yoshihiro Inoue, Seiichi Endo and Masami Tsuchiya, according to Japanese Justice Minister Yoko Kawakami.

Six other people are still sentenced to die in connection to the 1995 attack and other Aum Shinrikyo crimes. The date of their executions is not known.

Executions in Japan are done in secret, with no advance warning given to the prisoner, their family or legal representatives, according to Amnesty International . Prisoners often only learn hours before that they are to be killed.

Executions in Japan are carried out in secret and without warnings to prisoners' families or lawyers.

Shizue Takahashi, a victims group representative and widow of a Toyko Metro employee who died in the sarin attack, told reporters she was "surprised" by the sudden execution.

"When I think of those who died because of them, it was a pity (my husband's) parents and my parents could not hear the news of this execution," she said. "I wanted (cult members) to confess more about the incident, so it's a pity that we cannot hear their account anymore."

In a statement Friday, Amnesty said the execution of Asahara and other Aum Shinrikyo members would not deliver justice for the Tokyo attack.

"The attacks carried out by Aum were despicable and those responsible deserve to be punished. However, the death penalty is never the answer," said Hiroka Shoji, East Asia researcher at Amnesty International.

"Justice demands accountability but also respect for everyone's human rights. The death penalty can never deliver this as it is the ultimate denial of human rights."

Photo shows a gate of a Tokyo detention center on July 6, 2018, where Shoko Asahara, whose real name is Chizuo Matsumoto, was hanged for masterminding the 1995 sarin gas attack on the Tokyo subway system.

Doomsday beliefs

Asahara founded Aum Shinrikyo in 1984 and quickly attracted thousands of disciples , combining forecasts of a coming apocalypse -- which would come after the US attacked Japan and turned it into a nuclear wasteland -- with traditional religious teachings and new age tactics.

Many of Asahara's followers were highly educated scientists and engineers, who helped bring in huge amounts of money to the cult's coffers.

As the cult grew, the families of members began to raise the alarm, and complaints of brainwashing and abuse within Aum Shinrikyo became more common.

Despite this, few would have predicted what was to come, and the cult shot to global notoriety with the March 1995 attack on the Tokyo subway, when members of Aum Shinrikyo released sarin gas on carriages full of commuters during rush hour. The attack killed 13 people and injured 5,500.

Asahara and dozens of his followers were arrested in the months that followed, after police raids across the country.

At a press conference Friday, former Aum Shinrikyo spokesman Fumihiro Joyu said he carried a "heavy shared responsibility" for the group's crimes.

"I would like to apologize to the victims," he said. "I would like to work on compensation and to make sure such crimes never happen again."

Aleph, a successor group to Aum Shinrikyo, is still involved in a court case over settlements to the victims of the 1995 and other attacks. Joyu left Aleph in 2006.

He said that more than 10 years after he left the cult, he had "no special feeling" for Asahara, but had still been somewhat nervous about the potential repercussions for criticizing him in public. With Asahara's execution, he hoped this fear would go away.

Deadly cult

Aum Shinrikyo's killings began in November 1989, when lawyer Tsutsumi Sakamoto -- who was working on a class action case against the cult -- was brutally murdered along with his wife and child. The killing was eventually linked to the cult

Prosecutors said cult members entered the Sakamotos' home as they slept, injected them with lethal doses of potassium chloride and strangled them.

Sakamoto's murder and the growing clamor from cult members' families caused increased attention from the authorities, and Aum Shinrikyo began preparing for the end.

At a sheep farm in rural Western Australia and other properties, cult scientists began testing sarin while others synthesized the VX nerve agent and launched a failed attempt to manufacture automatic rifles.

On June 27, 1994, seven people were killed and more than 500 hospitalized after Aum Shinrikyo released sarin gas from a truck by driving slowly around an apartment complex in Matsumoto, Nagano prefecture. Another victim died in 2008.

Photo taken Jan. 7, 1990 shows Aum Shinrikyo cult group founder Shoko Asahara (fourth from left) speaking at a press conference in Tokyo to announce a plan to field candidates for the general election.

Subway attack

The Matsumoto attack was a warm up to the main event, which began almost eight months later on March 20, 1995, according to a report by the Federation of American Scientists.

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Five Aum Shinrikyo members boarded subway cars on three different lines in central Tokyo during rush hour, carrying plastic bags filled with sarin. They punctured the bags with the sharpened tips of theirs umbrellas and left them on baggage racks or the floor to seep the deadly gas into the carriages.

The trains were scheduled to arrive at central Kasumigaseki station within four minutes of each other, and the cult hoped not only to kill everyone on board, but also use the trains to deliver the gas to a massive interchange used by thousands of passengers at a time.

Fortunately, mistakes made in developing the sarin and its delivery method meant the attack was far less effective than intended, and the group only succeeded in killing 12 and injuring 5,500 people. Another victim died later.

According to the FAS report, chemical weapons experts estimate that "tens of thousands could have easily been killed" if the attack had been carried out correctly.

Shizue Takahashi, whose husband was killed by doomsday cult Aum Shinrikyo's sarin nerve gas attack while on duty at Tokyo Metro Kasumigaseki Station attends a memorial on March 20, 2018 in Tokyo, Japan.

Arrest and trial

Dozens of Aum Shinrikyo members were arrested after months of raids by police on hundreds of locations across Japan.

Asahara himself was arrested in May 1995, and indicted on 17 charges ranging from murder to illegal production of weapons and drugs.

His trial -- and appeals process -- took years to complete, and gripped Japan, as police continued to seek other cult members linked to the Tokyo and Matsumoto attacks.

In late 1996, Ashara admitted responsibility for the sarin attack but said he was not personally involved in the crime, saying he had been "instructed by God" to shoulder the blame. At the same time, he warned lawyers they would die if they continued questioning Aum Shinrikyo members.

After a trial that lasted eight years, Asahara was found guilty of masterminding the attack and sentenced to death in 2004. By 2006, he had exhausted the appeals process.

His execution was delayed due to ongoing cases against his co-conspirators, the last of whom was arrested in 2012

Aum Shinrikyo split into Hikari no Wa and Aleph in 2007, and the latter group has apologized for the Tokyo attack, which it blames on "top members of then Aum Shinrikyo." The two groups have around 150 and 1,500 followers respectively, according to Japanese media.