By Patrick J. McDonnell, Los Angeles Times (TNS)

PARIS — At a makeshift shrine down the street from a concert hall turned scene of carnage, a steady stream of well-wishers came Saturday to pay tribute to scores massacred when suicide assailants struck the music theater, a soccer stadium and several restaurants and bars.

“The French will combat these thieves of life,” declared a handwritten note.

But Friday’s brazen assaults — all targeting venues where people went to savor life in Europe’s storied cultural capital — served to dramatize anew the continent’s continued vulnerability to terrorism.

The string of shootings and bombings came less than a year after attacks here on the offices of satirical magazine Charlie Hebdo and a kosher grocery that left 17 dead. Last January’s strikes, while horrific, were aimed at specific targets — an irreverent publication that had lampooned Islam and a market that catered to Jewish shoppers.

This time, the victims appeared chosen simply because they were in France and were out on a weekend night to enjoy music, sports and the company of fellow human beings.

Preventing such attacks in a free society is a daunting security challenge, experts caution, even for nations like France with considerable experience fighting extremist groups and infiltrating militant cells.

“Paris is … a warning that the best counter-terrorist efforts in the world cannot protect any country, particularly the open societies in the West, from every attack,” wrote Anthony Cordesman of the Center for Strategic and International Studies, a Washington think tank.

French President Francois Hollande labeled the synchronized series of attacks an “act of war” that was “organized and planned from outside,” though details about the planning were publicly vague.

The extremist Islamic State group took credit for the attacks, which claimed more than 125 lives, boasting that it had targeted “the capital of prostitution and obscenity” in retribution for France’s involvement in the U.S.-led coalition bombing of extremist-held terrain in Syria and Iraq.

If Islamic State indeed carried out the attacks, Friday’s operation would represent the latest in a string of deadly operations outside of Syria and Iraq, the group’s home base. Its ability to carry out such sensational assaults would now appear to equal or even eclipse that of al-Qaida, a fierce rival.

Islamic State, a Qaida breakaway faction, has also taken responsibility for twin suicide bombings in a Beirut suburb last week that killed more than 40, and for downing a Russian commercial airliner over the Sinai two weeks ago, killing all 224 on board.

When it first emerged on the scene amid the tumult of the long-running Syrian war, many commentators said that Islamic State’s interests appeared to be largely local in nature. But that has changed considerably, perhaps in response to intense international efforts to destroy the group’s self-proclaimed “caliphate.”

In a chilling tone, Islamic State called Friday’s attacks “the first of the storm and a warning to those who wish to learn.”

Thus the Paris carnage may have been the latest collateral damage from the calamitous Syrian war, which has already destabilized much of the Middle East and helped generate a refugee crisis in Europe.

Friday’s attacks seemed certain to increase pressure on deeply divided world powers to help broker a political settlement to the Syria conflict — and to escalate military operations against Islamic State and other extremist groups that have thrived amid the chaos in Syria.

As investigators delve into the attack, there are many unanswered questions: Was it indeed conceived and planned abroad or in Europe? Where did the assailants get the guns, ammunition and explosives — and where did they assemble the bombs? Do any attackers or support personnel, such as bomb assemblers — remain at large? Police on Saturday were investigating whether one team of attackers may have escaped.

Several witnesses at the Bataclan concert hall mentioned hearing attackers say the words “Syria” and “vengeance.”

“I clearly heard them tell the hostages, ‘It’s the fault of Hollande, it’s the fault of your president, he shouldn’t have intervened in Syria,'” Pierre Janaszak, a radio and TV presenter, told the Agence France-Presse news agency. “They also spoke about Iraq.”

The operation demonstrated a considerable level of sophistication and probably involved weeks or months of surveillance and organization, authorities and experts said.

The use of heavily armed suicide or “kamikaze” attackers, as the media here referred to the assailants, was regarded as a first in France — though experts voiced concerns that the tactic, numbingly familiar in the Middle East, could become more common in Europe and elsewhere.

“Simplicity,” noted Marc Trevidic, a former French anti-terrorism judge, in an interview with BFMTV here. “There is nothing better when it comes to terrorism.”

France is considered particularly vulnerable to domestic terrorism. More than 1,800 of its citizens are believed to have traveled to Syria and Iraq to fight with extremists. Radicalized returnees trained in the Syria-Iraq battle zone present a significant threat, authorities say. But it was unclear late Saturday how many, if any, of the attackers in Friday’s killing rampage had traveled to Syria or Iraq.

The nationalities of the Kalashnikov-wielding assailants in the Paris attacks also remained a question mark. Witnesses at the Bataclan concert described the assailants as young, perhaps in their 20s, and speaking unaccented French, according to media accounts here.

One of the attackers was a 29-year-old Frenchman who had been investigated for “minor offenses” but had never been implicated in acts of terrorism, French public prosecutor Francois Molins said during a news conference.

Still, his involvement would appear to raise anew the question of whether an intelligence failure contributed to the tragedy — as many charged was the case in the January attacks. Those perpetrators were three French citizens, of Algerian and Malian origins, with varying previous links to Islamic extremist elements. All three eventually were killed in police shootouts.

In the aftermath of Friday’s attacks, a Syrian passport was found near the body of one of the three bombers who struck near Stade de France sports arena, where a soccer match between France and Germany was being held. But it was not immediately clear if the passport belonged to the attacker.

A Greek official said the passport found at the scene in Paris had been presented by a Syrian who crossed into the European Union via the Greek island of Leros in October. That again raised the worrisome specter that some small minority of the multitudes of migrants coming to Europe from Syria could be terrorist operatives.

(Special correspondent Nabih Bulos in Dubai, United Arab Emirates, and Times staff writers Alexandra Zavis and Michael Finnegan in Los Angeles and Richard A. Serrano, Bob Drogin, W.J. Hennigan, Brian Bennett and Paul Richter of the Tribune Washington Bureau in Washington contributed to this report.)

(c)2015 Los Angeles Times. Distributed by Tribune Content Agency, LLC.

French fire brigade members aid an injured individual near the Bataclan concert hall following fatal shootings in Paris, November 13, 2015. REUTERS/Christian Hartmann