The killer who knifed to death two police officials in their home this week, claiming allegiance to the Islamic State group, tracked the couple days before the gruesome murders and uploaded his claim of responsibility on the family computer, a judicial official said on Friday.

As the investigation into the gruesome deaths on Monday advanced, France honored the couple, Commander Jean-Baptiste Salvaing, No. 2 officer in Mureaux, a hardscrabble town west of Paris, and Jessica Schneider, a police administrator in nearby Mantes-la-Jolie, another challenging town for police.

Hundreds of officers convened for a somber ceremony at the prefecture of Versailles, the region where the two had lived and worked. President Francois Hollande, who presided, praised the couple as "two heroes of daily life."

Larossi Abballa, 25, convicted in 2013 of a role in a jihadi network for the Pakistan-Afghan border, was killed by a police intervention unit which had surrounded the home and tried in vain to negotiate.

The couple's 3-year-old son was found alive inside the home.

Signals from Abballa's recently purchased telephone were captured in the town where the couple lived, Magnanville, and around the Mureaux police station on three consecutive days before the June 13 killings -- on June 8, 9 and 10, the judicial official said.

"One can imagine that (he) was tracking" Salvaing, said the trusted official who was not authorized to discuss the case publicly.

The investigation revealed yet another macabre aspect to the killings: Abballa uploaded his more than 12-minute video claiming responsibility -- and pledge of allegiance to the Islamic State on Facebook Live -- using the family computer.

It also appears he took photos of Salvaing from the family computer, the official said.

The question of a revenge attack has hung over the killings. However, police officials have said there is no proof linking Salvaing to his killer, who lived in the area, even though the commander likely knew of him.   

France has been on tenterhooks about potential attacks by the Islamic State group after two waves of attacks last year, including November massacres in Paris that killed 130.   

Hollande said during the ceremony that security officials had stymied more than 15 attack attempts in the past few months.

The latest may be that of a 22-year-old Frenchman arrested this week in the tourist-heavy medieval city of Carcassonne, in southern France. He faces possible charges of criminal terrorist association for allegedly preparing to attack people in the street, notably Americans and Russians, the judicial official said.   

The man, from the Tarn region, north of Carcassonne, was detained this week with a knife and hammer after bragging online about wanting to kill people, a security official said Thursday. Further information, including his identity, was not immediately available.   

The killing of the police officials shook the nation's police officers, stretched by the state of emergency and the month-long Euro 2016 soccer championships.   

Investigators have yet to decipher the scenario of the killings, and are studying the possibility that the police officer's wife was killed first, contradicting initial reports, judicial and police officials said.

At the ceremony, Hollande said measures would be taken to ensure anonymity for police who now feel threatened out of uniform. He did not elaborate on the measures. He also said off-duty police may now carry arms, a demand that has grown with the threats.

"France will continue its implacable fight against terrorism with even more determination in memory of their sacrifice," Hollande told police mourning their colleagues, calling  police "sentinels of the Republic."

Hollande posthumously made Salvaing and Schneider, a police administrator, knights of the Legion of Honor, France's highest honor.