PARIS -- The Islamic State group's call on Muslims to go after the "filthy French" and other Westerners multiplies already deep security concerns in nations targeting the militant organization.
The appeal made public Monday makes intelligence tracking of potential suspects virtually impossible and opens up Muslims in the West to the possibility of being unfairly put under suspicion or stigmatized.
Nations are honing mechanisms to monitor Westerners who head to Syria and Iraq to fight in the jihad, the better to catch them when they return home with deadly skills. But how do you track someone who reads the Islamic State group's call in a newspaper or on a mainstream website, and then carries out a spontaneous attack?
Experts in terrorism agreed that the options to counter-act the call on all Muslims to kill are virtually nil, beyond bolstering security forces' visibility -- thus allowing them to act quickly if need be.
"We are not waging a war between east and west, or Christianity and Islam," French Prime Minister Manuel Valls said Tuesday. The French government says what it calls the "butchers" of the Islamic State group don't represent Islam.
But Valls acknowledged that France is facing an unprecedented challenge from "the enemy within."
"We have compatriots who could strike us," he said on Europe-1 radio.
On Friday, France became the first country to join the U.S. in carrying out airstrikes in Iraq. France, with the largest Muslim population in Western Europe, an estimated 5 million, also counts the highest number of citizens and residents who have turned to jihad in Syria and Iraq -- more than 900 people travelling or planning to go.
France has increased security around places of worship, airports and "symbolic" sites after the first airstrikes.
A French citizen captured Sunday evening in Algeria by a breakaway al Qaeda affiliate was the first victim of the new threat. A masked man crouching with the hostage in an authenticated video threatened his death if France doesn't end airstrikes on Iraq within 24 hours. The group, Soldiers of the Caliphate, said the kidnapping was a response to the Islamic State group's call.
On Tuesday, Australian police shot a man to death after he had stabbed two counterterrorism officers. It wasn't immediately clear whether the violence was related to the call from the Islamic State group, but police there said they were investigating reports that the deceased man had been waving an Islamic State flag.
The group's sweeping appeal in an audio statement implored Muslims to "not let this battle pass you by, wherever you may be."
The statement, issued by group spokesman Abu Muhammad al-Adnani, asked Muslims to use all means to kill a "disbelieving American or European -- especially the spiteful and filthy French -- or an Australian or a Canadian" or any disbeliever and others whose countries have joined to try to disable and destroy the Islamic State group.
Matthew Henman of IHS Jane's Terrorism and Insurgency Center said, "When you have people travelling out to Syria to fight, there are mechanisms in place that make it easy for security forces to track and survey those people ... when they return from the conflict zone."
But "all someone has to do is read a newspaper" reporting the threat and be inspired, he added. "It's extremely difficult for security forces to predict and intercept that because there's almost no intelligence."
Muslims in the West could become the collateral damage, stigmatized as potential extremists, as they have in the U.S. and Europe after attacks of the past. But this time they could fall under suspicion even if nothing happens.
The rector of the Grand Mosque in Lyon, which has a significant Muslim population, envisioned that possibility as soon as the Islamic State group's order went public.
Kamal Kabtane, along with two other Muslim leaders, said Monday the appeal risks creating an "anti-Muslim tsunami" and hands ammunition to those who "cast doubt on the loyalty of Muslim citizens regarding (French) values and democracy."
French Muslim leaders recently called for the nation's imams to use their pulpits against the Islamic State group, which has conquered wide territory in Syria and Iraq, where it was born under another name in murderous advances and displays of brutality like videotaped executions of two American journalists and a British aid worker.
Magnus Ranstorp, a specialist on asymmetric threats at the Swedish National Defence College, said that returnees and sympathizers would listen up most closely to the new appeal for Muslim support, and warned of a contagion effect.
"If there are instances like that it's the momentum that matters," Ranstorp said. "If you have an incident here and an incident there, you've got a problem. People imitate, people copy."
The UN Security Council is expected to adopt a binding resolution this week that would require nations to bar their citizens from travelling abroad to join extremist organizations. But it doesn't address what to do with radicals who stay at home but espouse the Islamic State group's goals. And officials in the Obama administration, which has championed the measure, acknowledge that it has no enforcement mechanism.
Even before this week's new threat, Westerners have pursued or aided jihad in Syria for a range of reasons. Americans among them include a nurse's aide who converted to Islam, a community college student with a Palestinian dad and Italian-American mom -- not people who would necessarily elicit suspicion.
France has already seen homegrown extremists take up arms close to home. A Frenchman, Mehdi Nemmouche, is the chief suspect in an attack on the Jewish Museum of Brussels in May that killed four people. And in 2012, Frenchman Mohamed Merah, who had trained on the Afghan-Pakistani border, killed seven people, including three Jewish children, three paratroopers and a rabbi in separate attacks in Toulouse.
More recently, Merah's sister is believed to have travelled to Syria. A French security official initially said her husband and two others were detained at Paris' Orly airport upon return from the region on Tuesday. But the Interior Ministry said in a statement late Tuesday that the three -- who had been held temporarily in Turkey -- were not in French custody.
In a breakdown in communication between Turkish and French authorities, the three were flown instead to southeastern Marseille and remained at large, according to the ministry statement. Authorities were still searching for the three, the security official said.
Claude Moniquet, a former agent of France's DGSE counter-intelligence unit and now head of Brussels-based European Strategic Intelligence and Security Center, said the ISIS appeal could also speak to people who suffer from emotional instability.
Moniquet pointed to a young French convert to Islam who attacked a soldier outside Paris days after a British soldier was hacked to death last year in London by suspected Islamist extremists. Psychological tests showed the Frenchman suffered from a range of emotional problems.
"It's too large a problem to be answered by intelligence services alone," Moniquet said. "It's a call for a kind of non-organized jihad: 'You can kill anyone ... and God will help."'
