— A 25-year-old Frenchman who fatally stabbed a police officer and his companion at their home in a suburb of Paris on Monday, an attack quickly claimed by the Islamic State, was detained from 2011 to 2013 for involvement in a terrorist network that aspired to be active in Afghanistan
and Pakistan, the authorities said on Tuesday.
At a news conference, the Paris prosecutor, François Molins, offered new details about the brutal killings and an ensuing standoff that lasted nearly four hours, ending when the police stormed the house, mortally wounding the assailant.
The killings shook France
, which has been on high alert since two major terrorist attacks last year and is struggling with the security challenges posed by the European Championship soccer tournament. They occurred just one day after a rampage at a gay nightclub in Orlando, Fla.
, by a gunman who had sworn allegiance to the Islamic State, left 49 people dead. And they raised new concerns about radicalization, as the assailant in France, identified as Larossi Abballa, was a French citizen who had been convicted and sentenced for terrorism-related activities.
The assault began between 8 and 8:20 p.m. on Monday, when Mr. Abballa fatally stabbed a police officer outside his home in Magnanville, a village about 35 miles west of Paris, according to Mr. Molins said. The officer, 42, worked in Les Mureaux, a town where Mr. Abballa had lived.
Mr. Abballa then entered the house and held hostage the officer’s companion, a 36-year-old woman who worked at a police station in the nearby small city of Mantes-la-Jolie, and the couple’s 3-year-old son, before Mr. Abballa fatally stabbed her.
A neighbor summoned the authorities, who included members of an elite police unit. According to Mr. Molins, Mr. Abballa told the police that he was a Muslim observing Ramadan; a supporter of the Islamic State and its leader, Abu Bakr al-Baghdadi; and that he was following an injunction by Mr. Baghdadi to “kill the infidels at home with their families.”
Mr. Abballa then broke off communications with the police. At some point during his time in the house, he turned his attention to social media. At 8:52 p.m., Mr. Molins said, Mr. Abballa posted a 12-minute video to more than 100 contacts, claiming responsibility for the attacks. He also posted two messages on Twitter from an account he opened last Wednesday.
Members of the elite police unit, known as RAID, stormed the home around midnight, fatally wounding Mr. Abballa and rescuing the child, who Mr. Molins said was traumatized but physically unharmed.
Inside the home, investigators found a list of possible targets — including rappers, journalists, police officials and other public figures — along with three phones, and several knives, including one with blood on it. In a parked car nearby, investigators found a Quran; a djellaba, a long robe commonly worn in North Africa
; and two religious texts.
Magnanville, where the victims lived, and Les Mureaux and Mantes-la-Jolie, where the victims worked, are all part of the Yvelines administrative department in the Île-de-France region, which includes Paris. Mr. Abballa was born in Meulan-en-Yvelines, also in that department.
Mr. Abballa was one of eight men convicted in Paris in 2013 of aiding a group that intended to commit terrorist acts and that had planned to go to Pakistan for training.
He was given a three-year sentence on Sept. 30, 2013, including a six-month suspended sentence. His sentence was further reduced, a common practice in France, by the fact that he had been in detention since May 14, 2011.
Although Mr. Abballa was released immediately after his trial, he was under surveillance for an additional two years and two months, until Nov. 30, 2015, Mr. Molins said.
“At the trial, Abballa seemed like someone who was not dangerous but was rather stupid,” said Hervé Denis, a lawyer who represented Zohab Ifzar, one of the other defendants.
Mr. Abballa had been unemployed at the time of his arrest, Mr. Denis said.
The police officer killed on Monday had nothing to do with the investigation that resulted in Mr. Abballa’s imprisonment, and Mr. Abballa never carried out his plan to go to Pakistan, although he had wanted to, Mr. Denis said.
“He appeared to be the perfect soldier,” Mr. Denis said — someone who would do whatever was needed to help the effort — and he was “very determined.”
Mr. Abballa was one target of a counterterrorism inquiry that Mr. Molins’s office began on Feb. 11, which focused on a conspiracy to commit terrorism.
As part of that inquiry, investigators have gathered intelligence from telephone intercepts and various geocoding technologies, but that intelligence did not indicate that Mr. Abballa was on the verge of violence.
Three men in Mr. Abballa’s circle — ages 27, 29 and 44 — have been arrested and held for questioning.
The attack in Magnanville has prompted calls in France for more stringent handling of those convicted of terrorism-related activities.
On early-morning broadcasts on Tuesday, some members of Parliament called for all people suspected of terrorist leanings to be placed in detention.
However, France is already stretched to the limit, both in prison capacity and in the numbers of suspects it can keep track of. The police and intelligence agencies are said to be working constantly to keep up with numerous would-be jihadists.
The antiterrorism directorate of the judicial police and of the internal security department of the Interior Ministry have detained 100 people this year on suspicions of links to terrorism, Bernard Cazeneuve, the interior minister, said early Tuesday. He added that the terrorism threat in France was “high.”
In testimony before members of the French Parliament on May 10, Patrick Calvar, the head of domestic intelligence, said that about 2,000 French citizens or residents were of concern to the security forces.
He testified that at least 645 French citizens or residents of France were in Syria
, and that about 400 of those were participating in military operations.
An additional 201 French citizens or residents are in transit, either traveling to Syria or returning to France from Syria or from Iraq, and 173 are presumed dead, Mr. Calvar said, adding that the number of dead was probably higher.
In addition, 244 French citizens have returned to France from Syria or Iraq
, and the French intelligence services say they have identified an additional 818 people who they believe want to join extremists.
The French authorities have repeatedly said that it is impossible to monitor all of them round the clock.