When an epidemiologist studied maps of shootings in Chicago, the pattern resembled the spread of infectious diseases.
At least 20 people were killed and more than 100 wounded when a gunman opened fire on a country music concert in Las Vegas.
Dozens of patrol vehicles descended on the famous Vegas Strip after reports of an “active shooter” near a country music festival late on Sunday.
“We have in excess of 100-plus injured and excess of 20-plus that have died at this point,” Las Vegas Metro Police Sheriff Joseph Lombardo told a press conference early on Monday.
The gunman, who was a local man, was killed after police “engaged the suspect”, Lombardo said. “The primary aggressor … is no longer a threat.”
Officers responded to reports of heavy gunfire coming from the 32nd floor of the Mandalay Bay, a hotel-casino next to the concert venue. Thousands of fans were attending the concert, which was part of a three-day country music festival known as Route 91.
Witness Cole Watson was at the concert with his wife and kids.
“At first, it sounded like someone was setting off a whole lot of firecrackers, but then we realised it was gunfire and everybody started running. The scene was insane – it was absolute chaos,” Watson told Al Jazeera.
At this time we do not believe there are any more shooters. More information to come shortly from @Sheriff_LVMPD.
— LVMPD (@LVMPD) October 2, 2017
Some officers took cover behind their vehicles, while others carrying assault rifles ran into the Mandalay Bay. Concert-goers reported seeing muzzle flashes from its upper floors.
Wade Millward, a reporter with the Las Vegas Review-Journal, said the attack happened while country music singer Jason Aldean was on stage performing, according to witnesses.
“He played for about 30 seconds before it dawned on everyone that this was not fireworks. People hit the ground; people ran,” Millward told Al Jazeera.
There were “dead people at the concert right next to me. We crawled over dead people,” witness Cari Copeland Pearson told dpa news agency.
Police were searching for a second female suspect believed to be a companion of the attacker.
“We have not located her at this time and we are interested in talking to her for follow-up,” Lombardo said. “It’s an ongoing investigation. Right now, we believe he is the sole aggressor at this point.”
Video posted shows a crowd of panicked people huddling together on the street outside of the hotel as automatic-weapon fire rang out.