A Syrian suspected of killing a tourist in the German city of Dresden early this month had been classified as a radical Islamist and was under observation on the day of the attack, security officials said on Thursday.
The 20-year-old man had been categorised in 2017 as likely to pose a threat to public safety, said Saxony state's head of police, Petric Kleine.
Some 600 suspected Islamists are in this category in Germany, according to authorities.
The Syrian was released from prison, where he had served more than two years for recruiting members to a terrorist organisation, only days before the attack.
In an assessment months before, authorities had said it was highly likely the man would commit more crimes once released, Kleine added.
He was under observation on the day of the attack and the two preceding days, said the head of the Saxony branch of the domestic intelligence agency, Dirk-Martin Christian.
He added that people just released from prison were not normally monitored 24 hours a day, but declined to give more details.
Investigators accuse the Syrian of attacking two tourists, one aged 53 and the other aged 55, from the western state of North Rhine-Westphalia with a knife. Both were severely injured and the 55-year-old later died.
The Syrian had come to Germany in October 2015 at the peak of the migrant crisis and was later granted refugee status, police said. He lost this status because of his criminal record in 2019, but could not be deported due to the civil war in his home country.