Police patrol at a street on the first working day after the truck attack in central Stockholm, capital of Sweden, on April 10, 2017. (Xinhua/Shi Tiansheng)

STOCKHOLM, June 19 (Xinhua) -- The Swedish Security Service (Sapo) is cracking down on potential terrorists among the population of rejected asylum applicants living underground in Sweden, local media reported on Sunday.

The April 7 attack in Stockholm prompted Sapo to step up efforts to find those who disappear when their asylum applications are rejected.

39-year-old Rakhmat Akilov driving a truck down one of Stockholm's busiest pedestrian streets killed five people. He was a rejected applicant living underground.

"We learned from this as soon as it happened," Anders Thornberg, head of Sapo, told Swedish public television broadcaster SVT.

Sapo is now looking for ways to work with police and the Swedish Migration Agency to detect potential threats.

"A deportation order can be very distressing. We want to try to find those individuals who could potentially commit a terrorist attack," Thornberg said.

Akilov had figured in Sapo's intelligence reports, but the case was dismissed when Sapo could not confirm suspicions.

We may need to modify our approach to this, Thornberg said. "We need to consider how to work with certain groups and criteria."

At the beginning of 2017, there were 12,500 rejected asylum applicants living underground and wanted by police. That number will likely increase in the coming years, SVT reported.

But how to capture those who pose a security risk is unclear.

"It's tough because it is also a matter of protecting rights. We need to find a vetting method and look into what more we can do," Thornberg said.

Another challenge for Sapo is the high number of asylum-seekers deemed a security threat to Sweden.

At the end of May, the Migration Agency had reported 353 potential security risks to Sapo. So far this year, Sapo has opposed granting residency in 19 asylum cases.