The federal prosecutor's office in Karlsruhe said a letter found in a getaway car claimed a Red Army Faction commando had carried out the bombings.
The leftist Red Army Faction guerrilla group has carried out numerous bombings, assassinations and other attacks against NATO, government and industrial targets in Germany over the past two decades.
The $153 million prison, 18 miles south of Frankfurt, had been scheduled to open the following Thursday. But the Hesse state justice minister, Christine Hohmann-Dennhardt, told reporters it would have to be torn down and rebuilt.
Police said the bombings caused $60 million in damage.
Five masked and armed people entered the prison complex, tied up the 11 workers and drove them to nearby woods. The attackers then detonated several bombs, destroying the administrative building and four cell blocks, said Hans-Juergen Foerster, federal prosecutor's spokesman.
Foerster said the attackers fled in a car owned by one of the prison employees and abandoned it at the town of Waldorf, nine miles north of Darmstadt.
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