French authorities were racing Sunday to hunt down any potential accomplices to the wave of terror attacks unleashed in Paris as the investigation widened beyond this nation's borders.

A French man believed to be directly involved in Friday's massacre in Paris is on the run and the subject of an international manhunt, French security officials said Sunday evening.

Investigators said the man rented a Belgian-registered black Volkswagen Polo, which was allegedly used and abandoned by the hostage-takers who killed 89 people inside a Paris concert hall.

He was identified by officials as Salah Abdeslam, 26, from Brussels.

Abdeslam is allegedly the brother of another suspect currently in custody and being questioned — and of one of the deceased attacker, officials said.

Three separate teams of terrorists armed with Kalashnikovs and identical explosives vests laid coordinated siege to Paris on Friday night, according to the Paris prosecutor. More than 100 people were killed at six locations and over 350 were injured.

French officials were working with authorities in Belgium, Spain and Serbia in an attempt to shed more light on the attack, which ISIS claimed responsibility for and which French President Francois Hollande described as an "act of war."

And the FBI has sent a "small contingent" of counter-terrorism investigators to Paris to augment its legal affairs representatives already there, the bureau said.

While officials have released a timeline of the attacks, authorities have offered little information on the attackers' identities.

Paris Prosecutor Francois Molins said seven terrorists died in the attack on Friday. Officials initially said there were eight attackers — as did ISIS. It seems now Abdeslam is believed to be the eighth attacker.

Among the seven dead attackers were a French national who had been radicalized and a Syrian born in 1990, according to Molins. Local French media partially identified two of the attackers as Ismail M. and Abbdul Akbak B, and a former French official confirmed that information to NBC News. The brother and father of Ismail M. were taken into custody Saturday, the official added.

That young man lived in Chartres at least until 2012, local lawmaker Jean-Pierre Gorge wrote on Facebook.

Meanwhile, French Justice Minister Christiane Taubira confirmed on NBC's "Meet the Press" that investigators were in contact with authorities in Spain and Belgium.

The European Parliament in Brussels said Sunday it was stepping up security and raising its terror threat alert level, a day after officials in Belgium arrested five people in connection with the attacks.

"We consider this means they have a network," Francoise Shepmans, mayor of the town of Molenbeek where the individuals were detained, told Belgium's TV RTBF.

The broadening scope of the investigation came as more signs emerged that the attackers may have infiltrated Europe as part of the influx of refugees flooding the continent — bringing the immigration debate raging on the continent back to the fore.

Serbia's Ministry of Interior confirmed to NBC News that a terror suspect of interest to French authorities was registered and requested asylum at the Presevo border crossing on October 7. The ministry would only give the suspect's initials — A.A.

And A.A.'s details matched those of an individual registered in Greece four days earlier, the ministry added, saying that it was cooperating with French security services.

Greek police earlier said at least one of the attackers was linked to a passport that was registered on the island of Leros.

A black market for passports in Syria is booming, and officials have been concerned that ISIS militants and other radicals might be infiltrating Europe as part of a flood of refugees hitting the continent.

Of particular concern is whether refugees are using falsified documents or stolen blank passports — especially given how ISIS acquired a virtually unlimited supply when it took over large swaths of Iraq and Syria in summer 2014.

While it was unclear if the passport registered on Greece was legitimate, it raised the specter of further backlash in Europe against the tide of refugees. Officials are struggling to cope with the influx and opposition to welcoming refugees has grown amid signs some cities are overwhelmed.

As investigators worked furiously to chase down leads, France continued to mourn the heavy toll. Prime Minister Manuel Valls said that as of Sunday morning, authorities had identified 103 of the 129 killed.

Valls met with relatives of victims at a counseling center set up at the historic Ecole MilItaire and told reporters there that all efforts were being made to offer support.

"Some families here are devastated, torn apart by pain," Valls told journalists after meeting with victims' loved ones. "Others know that their child has been killed but have no news at this stage because the body has not yet been identified."

A service was scheduled at Paris's famed Notre Dame Cathedral on Sunday and streams of people continued laying flowers and candles at impromptu memorials throughout Paris.

France's mourning was met with shows of solidarity across the globe.

Just as the Eiffel Tower went dark to demonstrate France's sorrow, so too did the Empire State Building. The New Orleans Superdome was among the U.S. landmarks illuminated with the colors of the French flag, while flags at the U.S. Capitol were lowered to half-staff.

Names of those who lost their lives also began to trickle out — along with tales of survival.

Julien Pearce, a French radio reporter who was at the Le Bataclan nightclub to watch the band Eagles of Death Metal, said concertgoers dropped to the floor and played dead as gunmen - three or four traipsed through the nightclub, he said - fired at random. After a few minutes on the ground, Pearce realized he would be killed if he didn't move. "We had to find an exit. At least to try," he said.