Burkhard Lischka, a parliamentary spokesman for the Social Democrats, said three of the nearly 550 individuals identified as Islamist threats in Germany could not be found.
Speaking after a meeting of the internal affairs committee in Berlin, Lischka held up as a warning the case of Anis Amri, a failed asylum seeker from Tunisia who fell off the authorities' radar despite long being regarded as a threat. Last month, the 24-year-old plowed a truck through a Christmas market in Berlin, killing 12 people and wounding dozens more.
"They are playing with fire, and every wrong calculation can be deadly," Lischka said.
Amri managed to flee to Italy, where he was fatally shot by police near Milan. The self-styled "Islamic State" terror group claimed responsibility for the Berlin attack, calling Amri a fellow "soldier."
The German government has been heavily criticized over the handling of Amri's case, with many questions raised as to why he wasn't apprehended. According to German media reports, investigators looking into his background ultimately decided it was unlikely he would carry out an attack.
Interior Minister Thomas de Maiziere on Wednesday announced that the country's Joint Terrorism Task Force would now carefully review the cases of each of the 547 people - both German nationals and foreigners - flagged as security risks to determine whether they should be deported or taken into custody.
According to authorities, around half of the Islamists considered potentially dangerous in Germany are currently living abroad. Domestic policy spokesman for the ruling coalition, Stephan Mayer, told the interior committee meeting that 88 of those individuals were in custody. More than a third come from the western state of North Rhine-Westphalia, while a significant number are also based in Berlin, Mayer said.
De Maiziere urged lawmakers to speedily approve a raft of new measures announced earlier this month to make it easier for authorities to take people into custody for deportation. He also said it was unacceptable that Islamist militants were able to move freely around the country, and called for guidelines to be set up to help authorities deal with those deemed potentially dangerous.
nm/gsw (Reuters, dpa)