Story highlights A married couple -- Dutch nationals of Turkish origin -- are arrested at Brussels Airport, official said

Three others arrested; one in Belgium, one in Turkey and one in the Netherlands

Suspects' links to groups in Syria are complex but point towards ISIS, official tells CNN

Five people, including a married couple, have been arrested in Europe on suspicion of terrorist activities, a senior Belgian counterterrorism official with knowledge of the case told CNN Sunday.

Belgian, Dutch and Turkish police launched a joint monthslong investigation that included extensive surveillance of a suspected terrorist cell with ties to jihadist groups in Syria, the official said.

"Their links to groups in Syria have been complex to untangle, but point towards ISIS," the official told CNN.

The couple -- Dutch nationals of Turkish origin -- were arrested August 6 at Brussels Airport on their way back from a two to three day trip to Istanbul.

A third suspect was arrested in Belgium. A fourth suspect was arrested in the Netherlands and a fifth was arrested in Turkey. Several of the alleged members of the cell, who traveled frequently between Belgium and the Netherlands, were of Turkish descent, the official said. All five remain in custody.

Authorities moved quickly to make the arrests after police learned the alleged cell had gotten handguns. The weapons were recovered in a raid on one of the suspects' Brussels residence on August 6. In the Netherlands, electronic media was seized during a raid in The Hague, including terrorist propaganda materials, the official said.

"It was obviously worrying when we discovered they'd obtained weapons," the official told CNN. "We were concerned they might be planning an attack here [in Belgium], but we still don't know what if, anything, they were planning."

The official said there was no evidence he knew of that the suspects were planning to attack the European Commission in Brussels, as some media reports in Europe have speculated.

Belgian security services have become increasingly worried about the threat from fighters returning from terrorist safe havens in Syria and Iraq. In May, Mehdi Nemmouche, a French ISIS fighter, allegedly killed four people at a Jewish museum in Brussels.

Nemmouche, who allegedly guarded French hostages while he was in Syria, was arrested in France after the shooting and extradited to Belgium to stand trial.

European officials believe about 2,000 EU citizens have answered the call to jihad in Syria, including the wives of fighters. Perhaps 500 to 1,000 of them have joined ISIS, officials say.

In June, a Belgian counterterrorism official told CNN that authorities were aware of 150 Belgian fighters in Syria, most of them with ISIS. Up to 15 more were leaving every month. About 35 are thought to have been killed, and 60 to 70 had returned home.

Round-the-clock surveillance is impossible for all but a very small number because of the prohibitive expense.