— -- At least three people have been arrested in Belgium -- including one officials say was involved in the attack -- as the investigation into the deadly Paris attacks has now spread across borders, officials told ABC News.
The investigation crossed borders after cars used in the coordinated attack were tracked to Brussels, officials said. One of the people arrested is believed to have been involved in the coordinated attacks Friday, and investigators continue to search for others.
A spokesperson for the Paris prosecutor told the Associated Press that family members of one of the attackers has also been detained. He is believed to be a French man born in the Parisian suburbs.
"Because they are looking for suspects, it was important to conduct operations in Brussels because of the ties they could potentially have here," an official told ABC News.
The operations are being conducted in the Molenbeek district of Brussels, the official said.
Belgian Justice Minister Koen Geens told a Belgian radio station that authorities descended on Brussels after tracking down a rental car that was spotted outside a concert hall in Paris, the site of the bloodiest of attacks.
Paris prosecutor François Molins said seven “terrorists were killed during their criminal actions.” Earlier, terrorist organization ISIS claimed responsibility for the attack, and said eight “brothers” took part in the operation, the same number of attackers seen in early local media reports.
Daniel Benjamin, a former counterterrorism coordinator at the U.S. State Department, said Belgium has been grappling with a “serious jihadi issue” for more than a decade. The country has the dubious distinction of seeing more residents, per capita, traveling to Syria to fight with ISIS, he said.
“My impression is that Belgian authorities have been taking the threat more seriously as time as gone by,” Benjamin said. “But I don’t believe they have the capacities of the British or the French.”
Benjamin said the United States has been urging a number of European countries to increase their spending on police, surveillance and intelligence, especially in the aftermath of the Charlie Hebdo attacks in Paris in January.
“This will probably be a pivot point,” said Benjamin, who is now the director of the John Sloan Dickey Center for International Understanding at Dartmouth College.