PARIS — 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, authorities said 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 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 in the United States, at a gay nightclub in Orlando 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. Monday, when Abballa fatally stabbed a police officer outside his home in Magnanville, a village about 35 miles west of Paris, according to Molins said. The officer, 42, worked in Les Mureaux, a town where Abballa had lived.

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 Abballa fatally stabbed her.

A neighbor summoned authorities, who included members of an elite police unit. According to Molins, Abballa told 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 Baghdadi to “kill the infidels at home with their families.’’

Abballa then broke off communications with police. At some point during his time in the house, he turned his attention to social media. At 8:52 p.m., Molins said, 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 Abballa and rescuing the child, who 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 Koran; 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. Abballa was born in Meulan-en-Yvelines, also in that department.

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 Abballa was released immediately after his trial, he was under surveillance for an additional two years and two months, until Nov. 30, 2015, 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, another defendant.

Abballa had been unemployed at the time of his arrest, Denis said.

The police officer killed Monday had nothing to do with the investigation that resulted in Abballa’s imprisonment, and Abballa never carried out his plan to go to Pakistan, although he had wanted to, Denis said.

“He appeared to be the perfect soldier,’’ Denis said — someone who would do whatever was needed to help the effort — and he was “very determined.’’