By Chine Labbé and Simon Carraud
PARIS/LES MUREAUX (Reuters) - A suspected Islamist attacker stabbed a French police commander to death outside his home and later killed his companion, a policewoman, in an attack claimed by Islamic State and denounced by the government as "an abject act of terrorism".
The assailant, a 25-year-old Frenchman of Moroccan origin, was jailed in 2013 for helping Islamist militants go to Pakistan and had been under security service surveillance, including wiretaps, at the time of the attack, police sources said.
The attacker filmed part of the assault live on the social network Facebook, according to David Thomson, a journalist specialized in radical Islamists. In his Facebook message, he linked the attack to the Euro 2016 soccer tournament now under way in France, saying: "The Euros will be a graveyard."
The attacker, named by police and justice sources as Larossi Abballa, knifed the 42-year-old commander repeatedly in the stomach on Monday evening.
He then barricaded himself inside the house in Magnanville, a suburb some 60 km (40 miles) west of Paris, taking the man's 36-year-old partner and their three-year-old son hostage.
Police commandos shot Abballa dead when they stormed the house after negotiations failed but they found the woman, a secretary at a police station in a nearby suburb, killed with a knife, a source close to the investigation said.
The boy was unharmed but in a state of shock.
"An abject act of terrorism was carried out yesterday in Magnanville," Interior Minister Bernard Cazeneuve said after an emergency government meeting, before visiting Les Mureaux, where the police commander worked.
President Francois Hollande said the killings were "undeniably a terrorist act" and that the terrorist threat in France was very high.
Police searched Abballa's home and other locations on Tuesday and detained two people close to him for questioning, a police source said.
The killings came as France, which has been under a state of emergency since Islamic State gunmen and bombers killed 130 people in Paris last November, was on high security alert for the Euro 2016, which began last Friday.
Police are under "extreme pressure" and "close to burn-out," the head of FO labor union Jean-Claude Mailly told France 2 television.
Islamic State claimed the attack. "God has enabled one of the caliphate's soldiers in city of Les Mureaux near Paris to stab to death the deputy police chief and his wife," an official broadcast on its Albayan Radio said.
If it is confirmed that the group was behind the killing, it would be the first militant strike on French soil since the multiple attacks on bars, restaurants, a concert hall and the national soccer stadium in Paris in November.
Details started to emerge on the profile of the attacker. Abballa was born in the nearby town of Meulan and lived in Mantes-la-Jolie, where he had set up a fast food outlet in April, documents from the Versailles court showed.
He was given a three-year prison sentence in 2013 for helping Islamist militants go to Pakistan. His name appeared in a separate ongoing investigation into a man who went to Syria, but he was not considered a threat, a source close to the probe said.
"He wanted to do jihad (holy war), that was clear," Marc Trevidic, a former anti-terrorism judge who was in charge of the 2013 investigation told Le Figaro newspaper. But he was seen as having a minor role in that case, he said.
Abballa had also been convicted three times on charges of aggravated theft and driving without a license, a source close to the investigation said.
"Many things are being analyzed," a justice source said, including messages posted on social networks.
Thomson, an RFI radio journalist specialized in Islamic radicalism, wrote on his Twitter page that Abballa had filmed himself on Facebook live during the attack.
With the couple's boy behind him he said: "I don't know yet what I'm going to do with him," Thomson wrote.
Islamic State's claim of responsibility came after the Islamist militant group said it was responsible for the shooting that killed 49 people in a massacre at a gay nightclub in Orlando, Florida.
(Additional reporting by Leigh Thomas, Marc Joanny, Matthieu Rosemain, Richard Lough in Paris and Muhammad Yamany in Cairo; Writing by Ingrid Melander; Editing by Paul Taylor and Dominic Evans)