PARIS — In an amateur video that was widely circulated online after the shooting Wednesday at the Charlie Hebdo newspaper offices in Paris, two gunmen could be seen killing a police officer as he lay wounded on the ground.

“We have avenged the Prophet Muhammad!” they shouted as they walked back to their getaway car parked nearby. “We have killed Charlie Hebdo!”

The officer, Ahmed Merabet, was the second police officer to be killed that day; another officer assigned to protect Charlie Hebdo’s editorial director was also killed. On Thursday, a third police officer was killed in a seemingly unrelated shooting south of Paris in another blow to the police forces here.

Image Ahmed Merabet

But the killing of Officer Merabet stood out: The #JeSuisAhmed hashtag — “I am Ahmed” in French — sprang up on social media alongside #JeSuisCharlie, as users of social media stood up for the slain officer.