But as police officers still considered raiding the apartment, viewers could already watch a disturbing online video filmed by the attacker inside the house.
Before he was eventually shot by police, the attacker, identified as Larossi Abballa, 25, swore allegiance to the Islamic State in the live-streamed video, according to the Associated Press.
Monday's video should raise worrisome questions about the future of so-called lone wolf attacks by radicalized individuals and their use of social media. It is not the first time Islamic State fighters or sympathizers have filmed themselves or others during assaults — and will likely not be the last.
Although the footage, as well as the Facebook account it was posted on, were removed shortly after the attack, the photos and the video continued to circulate online. David Thomson, a journalist with Radio France Internationale and an expert on Islamist extremism, explained that some of the images showed the two victims.
In his video, the attacker also said: "I'm not yet sure what to do with him," referring to the stabbed couple's son, who appeared to be hiding behind a sofa. The 3-year-old remained unharmed.
"I just killed a police officer and his wife," the assailant reportedly said in the video, in which he also warned that the current Euro 2016 soccer tournament "would be a graveyard."
One element that distinguishes terrorism from other types of crimes is the fear such attacks are supposed to cause among the wider public. Videos have proven to be an effective way for groups to both recruit new members and to intimidate opponents.
The use of videos by terror groups has a long and gruesome history: Former al-Qaeda leader Osama bin Laden spread his messages using audio or video tapes, mainly on television. The 9/11 attacks were set up in a way that would create the worst photos and videos possible, broadcast live around the world.
Other terror groups have sought to imitate al-Qaeda. More recently, the Islamic State published a series of propaganda videos, showing executions or disseminating threats directed at Western nations. Following the Paris attacks in November, the group released a video allegedly showing some of the attackers during their time in Syria.
The Islamic State's videos did not always achieve their intended goals, though: Headcam clips filmed by fighters in Iraq recently showed a disorganized and chaotic attack and proved that the organization is far from being a professional army on the battlefield.
It is also not the first time that a single attacker living in a Western nation — inspired but not directed by a group — used videos during police standoffs. In 2014, for instance, an Islamic State-sympathizer in Sydney reportedly forced some of his hostages to read his demands in a video addressed to the Australian government.
Sharing footage of an ongoing attack on social media is still far less common than more traditional propaganda videos. But Monday's video should nevertheless worry authorities: Live-streamed content might enable assailants to portray themselves as role models for other lone wolves in Western countries and could motivate them to imitate attacks.
Videos like the one that emerged from France on Monday night might give lone wolves what they crave the most: the applause of other radicalized individuals or groups and the attention of an audience that is watching in horror.
Perhaps even more importantly, they demonstrate how helpless authorities appear to be in such situations. As viewers watched the assailant wondering whether to kill the couple's 3-year-old son, all French police could do was ask the public not to share the video.