Brokstedt's alleged perpetrator was until recently still in a Hamburg correctional facility (JVA).
The reason was a bodily harm, said the police department in Itzehoe, Schleswig-Holstein, on Thursday.
According to dpa information, the man was housed in the Hamburg JVA Billwerder.
Most recently, the stateless Palestinian was without a permanent address, according to the police.
Itzehoe/Hamburg – The 33-year-old is said to have stabbed passengers with a knife on the regional train from Kiel to Hamburg on Wednesday while traveling.
A 16-year-old and a 19-year-old died.
Several people were injured.
The alleged perpetrator was eventually overpowered by other passengers and arrested by the police at Brokstedt station.
dpa