BERLIN -- German officials had deemed the Tunisian man being sought in a manhunt across Europe a threat long before a truck plowed into a Christmas market in Berlin -- and even kept him under covert surveillance for six months this year before halting the operation.

Now the international manhunt for Anis Amri -- considered the prime suspect in Monday's deadly rampage -- is raising questions about how closely German authorities are monitoring the hundreds of known Islamic extremists in the country.

The issue puts new pressure on Chancellor Angela Merkel, who is running for re-election next year. Critics are lambasting her for allowing hundreds of thousands of asylum-seekers to enter the country, allegedly without proper security checks.

Among them was Amri, a convicted criminal in both Tunisia and Italy with little chance of getting asylum who successfully evaded deportation from Germany even as German authorities rejected his asylum application and deemed the 24-year-old a possible jihadi threat.

He is suspected in the attack that left 12 people dead and 48 injured Monday evening in Berlin. Health officials said 12 of the injured had very serious wounds.

After German media published photos of him and a partial name, federal prosecutors issued a public appeal for information along with the promise of a $105,000 reward for his arrest.

Authorities initially had focused their investigation on a Pakistani man detained shortly after the attack but released him a day later for lack of evidence. After finding documents belonging to Amri in the cab of the truck, they issued a notice to other European countries early Wednesday seeking his arrest.