A man who was shot was in critical condition, the police said, having been shot four or five times, but they had not yet been able to learn from him whether he was the man in the video. Law enforcement authorities said the wounded man was 30 and a known gang member who had been released from prison not long ago.

Some experts say Chicago’s uptick in shootings and killings may be an anomaly, but it has caused deep concern here. Officials said many of the shootings involved the city’s increasingly splintered web of gangs, with a growing number of disputes now playing out on social media sites — the equivalent of gang graffiti from another era.

Not long after the shooting on Thursday, the police said they found the video on Facebook during what has become a routine search of social media in such investigations. As soon as calls of shootings and other violence come in, police analysts begin scanning for related chatter, videos and postings. “That’s in our playbook at this point — we canvass on the street and we canvass digitally,” Mr. Guglielmi said.

The rise in violence comes at an especially difficult time: The Police Department is being scrutinized by the Justice Department; a new interim police superintendent took over the department this week; and residents, especially African-Americans, have voiced distrust in the police after the release of a video showing a black teenager named Laquan McDonald being shot 16 times by a white police officer in 2014.

As of Sunday, Chicago had reported 133 murders in 2016, compared with 77 in the first three months of last year. And shootings have increased by 91 percent from the same period a year ago.