BERLIN (AP) — A convicted far-right murderer temporarily took two prison guards hostage in eastern Germany during an attempted jailbreak before he was overwhelmed by other guards.

The hostage taker, who was convicted to life in prison for fatally shooting two people after he tried to storm a synagogue in the eastern German city of Halle in 2019, took two guards hostage at the Berg high-security prison near the city of Magdeburg in eastern Germany on Monday night.

In under an hour, eight others guards freed their colleagues and overwhelmed the attacker, 30-year-old Stephan Balliet, who was injured during the incident. The two hostages were not injured.

State security officials in Saxony-Anhalt, where the prison is located, said the prisoner used a “device” to force the two prison guards to take him out of his cell. They refused to describe the object due to the ongoing investigation.

Based on what the prisoner can be heard saying on surveillance footage, the officials said they assume Balliet was trying to escape from the prison.

In 2020, while he was still on trial, Balliet tried to escape from another prison. He climbed over a 3.40-meter-tall (11-feet-tall) fence during a yard exercise and spent five minutes looking for ways out of the prison complex before guards caught him, German news agency dpa reported.

After Monday’s incident, Balliet was placed in a special security room where he is under constant surveillance, and he may soon be transferred to another high-security prison, said Franziska Wiedinger, Saxony-Anhalt’s state minister for justice.

Germany’s top security official called on the country’s prison operators to review their security measures.

“This worries me, of course,” Interior Minister Nancy Faeser told Welt Television. “This calls on those who are responsible for prisons in Germany to take another very close look. Especially with someone like the assassin from Halle, who we know already tried to escape during the trial, there needs to be special attention.”

Balliet attacked the synagogue in Halle on Yom Kippur, Judaism’s holiest day, in 2019. He was armed with multiple firearms and explosives. When he didn’t succeed in entering the building, he killed a passer-by and a man inside a nearby kebab store.

The attack was one of the most violent and overtly antisemitic acts in postwar German history. Balliet was sentenced to life in prison in 2020.