The ruthlessness of the attack and the nature of the target -- a publication known for ridiculing Islam and other religions -- suggested possible ties to a radical organization, U.S. officials and others said. But no group has asserted responsibility, and it remained possible that the assailants were homegrown radicals without any direct ties to groups such as al-Qaeda or the Islamic State.
At least one of the men identified by the French authorities as one of the suspected assailants, Cherif Kouachi, a 32-year-old French citizen, was given a three-year sentence in 2008 for associating with a terrorist group because he was planning to go to Iraq to fight U.S. forces. Kouachi was arrested in 2005, and his lawyers said at the time that he had had second thoughts and was relieved he was stopped before leaving France, according to a report in The New York Times.
His brother, Said Kouachi, 34, was also identified as a suspect in Wednesday's attack, along with an 18-year-old, Hamyd Mourad.
Officials said the attack, involving military-style rifles and vests designed to carry ammunition, offered the appearance of some planning.
"The way these men moved and executed these terrorist attacks shows that they have not been amateurs," an Arab intelligence official said. "It was like a commando operation."
A U.S. official briefed on the intelligence surrounding the attack offered a similar assessment. "So often, the homegrown extremists have been kind of bumbling idiots," the official said, referring to planned attacks on Times Square and other locations. Those returning from Syria "know what they're doing. They have been through it before and can operate under pressure and operate very lethally."
Others were not convinced that the Paris attackers had professional training. While the attack, as seen on video clips, seemed well orchestrated, the shooters at one point cross each other's paths as they advance up the street - a type of movement that professional military personnel are trained to avoid as it would limit the ability of the shooters to maximize firepower.
(Firefighters and police officers gather in front of the offices of the French satirical newspaper Charlie Hebdo in Paris on January 7, 2015 and treat victims, after armed gunmen stormed the offices leaving eleven dead, including two police officers, according to sources close to the investigation. AFP PHOTO / PHILIPPE DUPEYRAT)
"From what I've seen, their shooting stance and movement indicates they are not well trained," Dan Rassachack, a Marine with training in close-combat skills, said in an email.
U.S. and Western security officials cautioned that they had not reached any conclusions about the affiliation of the three attackers, who killed 12 people in the assault, or determined whether they had any military training or had been to Syria to fight there.
Instead, they cited a range of possible links or inspirations, including al-Qaida or its regional affiliates, and a pair al-Qaida offshoots, the Islamic State and Jabhat al-Nusra, that have also turned the Syrian battleground into a competition for Islamist primacy.
U.S. Rep. Adam Schiff, D-Calif., said that a briefing delivered to members of the House Intelligence Committee on Wednesday outlined an array of scenarios, including that the attack was "inspired by ISIS or was a command and control decision" executed by the group. ISIS is an acronym for the Islamic State.
"I think, given the nature of the attack, we have to be very concerned that this is another devastating indication of the problem we're going to have from foreign fighters," Schiff said, referring to the more than 15,000 foreign militants who have flocked to the conflict in Syria over the past four years, including at least 3,000 from Europe.
"We don't know yet if that is the case," Schiff said. "But if it is, it will be some of our worst fears materialized."
France has seen as many as 1,000 of its own citizens depart to fight in Syria, with most of those doing so over the past year as part of an accelerating flow, French officials have said. France is also regarded as the most aggressive nation in Europe in monitoring suspected terrorist groups and seeking to disrupt the flow of fighters to Syria by taking measures including seizing passports.
A French citizen, Mehdi Nemmouche, last year carried out the first attack by a returning Syria militant when he crossed into Belgium and killed four people at a Jewish museum. In March 2012, a gunman claiming links to al-Qaida killed three Jewish schoolchildren, a rabbi and three paratroopers in Toulouse in southern France.
France had not seen as deadly a terrorist attack on its soil in nearly two decades before the masked gunmen burst into the offices of the French newspaper Charlie Hedbo on Wednesday and then escaped by car into the Paris suburbs.
After Wednesday's attack, militants affiliated with the Islamic State recirculated a video that was recorded and released last year calling for so-called "lone wolf" attacks in France.
Though there was some speculation that Wednesday's attack involved al-Qaida's affiliate in Yemen, some experts noted the absence of the hallmarks typically associated with an al-Qaida plot.
"It' a pretty unusual attack from a jihadist organization. It's not a suicide attack, and it's not a bombing," said Daniel Benjamin, the former top counterterrorism official at the State Department. "We haven't seen many cases of cells affiliated with [al-Qaida-linked] groups carrying out shootings like this."
A link to the Islamic State, even if the assailants were only inspired by the group and not ever formally part of its ranks, would be particularly alarming to officials in Europe, Benjamin said.
"The appeal of ISIS is sufficient that it is drawing out people who were not known as extremists before," said Benjamin, now at Dartmouth College. "Its efforts to create something like a state appears to have real appeal at the street level and has changed the terms of the game in radicalization."
U.S. and European officials said the Paris attack was likely to intensify pressure on security officials there to expand surveillance of Islamist groups, crack down on militants suspected of planning to leave for Syria and shore up airport screening measures that do not allow European citizens to be subjected to the same scrutiny as travelers entering the United States.
Although the attack in Paris was deadlier than other recent cases, Schiff said it should be seen as a part of an emerging pattern that also has included violent attacks in Canada, Australia, London and Belgium.
"There's a broad pattern of a proliferation of these one-off attacks around the world," Schiff said. "It's becoming an endless parade of brutality."
