Greek police said Saturday that they had detained a group of suspected terrorists and were trying to determine links between them and a cell of Islamic militants in Belgium.
The operation followed an urgent request that Belgian authorities relayed to Athens a day after a police raid in the east of the country resulted in the deaths of two purported militants whom authorities suspected of planning an imminent and substantial attack on police.
"The investigation is still underway," said Greek police spokesman Kleanthis Papagiannopoulos. "We are trying to verify whether any or all of these people are actually the suspects the Belgian authorities have alerted us about."
Papagiannopoulos refused to disclose how many suspects had been detained, though local media said a police sweep in central Athens resulted in the roundup of four men, including a man appearing to be Abelhamid Abaaoud, the suspected ringleader of the foiled attack in Belgium.
Since Thursday's deadly raid in Belgium, authorities there have arrested more than a dozen people, including several who had returned from trips to Syria. Federal magistrate Eric Van der Sypt said the suspects were within hours of implementing a plan to stage "large-scale attacks" targeting police.
Belgian officials on Saturday began deploying hundreds of troops to patrol streets in the wake of Thursday's sweep.
Greek news outlets said DNA tests along with photos and fingerprints of the suspects netted in Athens had been sent to Belgium for verification. They said all four of the suspects were of Moroccan descent.
Abaaoud, 27, the son of a businessman, was raised outside Brussels in the city of Sint-Jans-Molenbeek, where some of the suspects were searched and arrested in overnight counter-terror raids Thursday that killed two suspects and wounded a third after a firefight in the eastern city of Verviers.
Police were apparently led to Abaaoud by suspicious telephone calls between an inmate at Belgium's Liege Lantin prison and a militant in Greece who had returned from the fighting in Syria. That inmate also maintained regular contact with his brother, one of the two extremists killed Thursday in Verviers, according to Radio Television Belge Francophone, or RTBF.
Abaaoud is said to have appeared with other Islamic State fighters in an online video driving a green pickup truck with a pile of bodies tied to the back, dragging them through a field.
According to Paris Match, the video was filmed in the northern Syrian town of Hraytan on Feb. 12, when 90 people were massacred by Islamic State.
In the video, Abaaoud sits behind the wheel of the truck, observing that "before it was towing jet skis," motocross bikes and trailers loaded with luggage and gifts.
"Now you can film my new trailer," he says, as another man pans to the pile of bodies, shouts in Arabic, "God is great!" and chuckles.
Abaaoud is also believed to have recruited his 13-year-old brother - shown posing with a rifle in an online video - to fight for Islamic State in Syria, according to RTBF.
Up to 300 soldiers were being deployed this weekend in Brussels and the northern city of Antwerp, which has a large Jewish population, and may eventually be sent to Verviers, Belgian Prime Minister Charles Michel's office said in a statement.
"The mobilized troops will be armed and their primary responsibility will be to survey certain sites" and to support police, the statement said.
Greece, wedged between the West and the realm of Islam, could become a conduit for militants traveling in and out of Western Europe, intelligence experts say.
In recent months, Greek intelligence detected at least six Islamic State recruits traveling through the country, including a French citizen carrying a computer memory stick with instructions for making bombs.
Belgium has become a major recruiting ground for Islamic militants in Syria and Iraq. Authorities suspect that as many as 450 Islamist fighters have been recruited from the tiny state.