PARIS (AP) - French police hunted Tuesday for a second terrorist believed to have escaped after the bomb and gun massacres in Paris, while a U.S. official revealed that the suspected mastermind was part of an Islamic State cell that American intelligence agencies had been tracking for months.

Meanwhile, France and Russia unleashed a new wave of airstrikes against IS targets in Syria, while fears of further terror attacks deepened in Paris and beyond. The Eiffel Tower closed to the public just a day after it had reopened and a soccer match between Germany and the Netherlands was canceled due to a bomb threat just 90 minutes before kickoff.

Islamic State militants have claimed responsibility for the Friday the 13th attacks that targeted France's national soccer stadium, a packed concert hall and popular restaurants and cafes in one of Paris' trendiest neighborhoods, killing 129 people and wounding more than 350.

French authorities had previously said that at least eight people were directly involved in the bloodshed Friday: seven who died in the attacks and one who got away and slipped across the border to Belgium. However, there have been gaps in officials' public statements, which have never fully disclosed how many attackers took part in the deadly rampage.

On Tuesday, officials disclosed to The Associated Press that they now believe at least one more attacker was involved than was previously known, and they were working to identify and track down that suspect. Three officials spoke on condition of anonymity because they were not authorized to provide details about the ongoing investigation.

A new surveillance video obtained by the AP indicated that a team of three attackers carried out the shootings at one of the cafes. The video was among evidence authorities used in concluding that at least one other attacker was at large, the French officials indicated.

The brief clip shot from a distance shows two black-clad gunmen with automatic weapons calmly firing on the bar then returning toward a waiting car, whose driver was maneuvering behind them. Authorities believe that car is the same black Spanish-made SEAT vehicle that was found abandoned Saturday with three Kalashnikovs inside. In the footage, as the gunmen fire their rifles, patrons can be seen falling down on the pavement as others dashed or crawled away. The encounter lasted only seconds.

Previously officials had not specified how many people were involved in the attack on the sidewalk bar on La Fontaine au Roi street, as well as the other night spots.

In all, six attackers died after detonating suicide belts and one was killed by police gunfire. A manhunt has been mounted for an eighth suspect, Salah Abdeslam, whom French police accidentally permitted to cross into Belgium on Saturday. One of his brothers, Brahim, was among the attackers who blew themselves up in Paris. Another brother, Mohamed Abdeslam, was arrested after the attacks but released. In an interview Tuesday with French TV station BFM, he urged his brother to turn himself in.

Two men arrested in Belgium, meanwhile, admitted driving to France to pick up Salah Abdeslam early Saturday, but denied any involvement in the attacks, their lawyers said.

Mohammed Amri, 27, and Hamza Attou, 21, are being held on charges of terrorist murder and conspiracy. Belgian media reported the two were being investigated as potential suppliers of the suicide bombs used in the attacks, since ammonium nitrate, a fertilizer that can be used to make explosives, was discovered in a search of their residence.

More details emerged about the brothers' activities leading to the attack. Salah and Brahim Abdeslam booked a hotel in the southeastern Paris suburb of Alfortville and rented a house in the northeastern suburb of Bobigny several days before, a French judicial official told AP, speaking on condition of anonymity because she was not authorized to speak about the ongoing investigation.

Austria's Interior Ministry also disclosed that Salah Abdeslam had entered Austria about two months ago with two unidentified companions.

Officials have identified 27-year-old Abdelhamid Abaaoud, a Belgian of Moroccan descent, as the mastermind of the attacks. He is believed to be in Islamic State-held territory in Syria.

A U.S. official briefed on intelligence matters said Abaaoud was a key figure in an Islamic State external operations cell that U.S. intelligence agencies have been tracking for many months.

Analysts have been debating to what extent the Islamic State group was devoting resources to external terrorist attacks, versus seeking to hold onto the territory it has seized in Iraq and Syria. A consensus is now emerging that the group is more focused on exporting terror than had been widely understood.

U.S. intelligence agencies have some insights into who is involved in that effort, said the official, who was not authorized to discuss the matter publicly and spoke on condition of anonymity. But their understanding is hampered by the many challenges of gathering intelligence in Syria, where the CIA does not have a regular presence on the ground

The chairman of the Senate intelligence committee said Tuesday that it was likely that militants plotting the attacks in Syria, Belgium and France used encryption to hide their communications from authorities. Sen. Richard Burr, R-North Carolina, said there was no direct evidence of encryption, but that authorities had concluded it was used because they have uncovered no evidence of conversations among the plotters.

The statement acknowledges the challenges intelligence agencies face in monitoring records of international phone traffic.

Speaking to reporters after a classified intelligence briefing, Burr also said there is a "strong likelihood" the Paris attacks were directed, rather than just inspired, by the Islamic State group in Syria.

The comments were the strongest public attribution yet by American officials. CIA director John Brennan said Monday the attack bore "the hallmarks of terrorism carried out" by the Islamic State group.

Tensions remained high in much of Europe in the wake of the attacks. In Paris, the Eiffel Tower shut down again after opening for just a day Monday, and heavily armed troops patrolled the courtyard of the Louvre Museum. In the German city of Hannover, an exhibition soccer game between Germany and the Netherlands was canceled at the last minute and the stadium evacuated by police because of a bomb threat. Top government officials, including Chancellor Angela Merkel, had been scheduled to attend the game as a sign of defiance.

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Associated Press writers Karl Ritter, Matthew Lee and Jill Lawless in Paris, Ken Dilanian in Washington and Raf Casert and Lorne Cook in Brussels contributed to this report.