A rooftop gunman has killed six people in a "very random" yet "very intentional" shooting at an Independence Day parade in Chicago.
Hours later, police finally had a suspect in custody but the latest mass shooting, they said, had left Americans "shaken to our core".
Soon after, two policemen were shot in the city of Philadelphia, as thousands gathered for a July 4 concert and fireworks show.
Here's how it all unfolded.
What happened at the Chicago parade?
The gunman was on the roof of a business in the wealthy suburb of Highland Park when he opened fire on the parade at about 10:15am local time (1:15am AEST).
Police said an unsecured ladder in an alley had allowed him to access the roof.
Highland Park Police Commander Chris O’Neill said the gunman apparently used a "high-powered rifle" to fire from a spot where he was "very difficult to see".
Videos show families walking through the streets before hearing sirens as word filtered back about the shooting, then running to get their kids off the streets.
Dozens of bullets were fired, sending hundreds of parade-goers — some visibly bloodied — fleeing.
It happened about three-quarters of the way through the parade, and at a prime place on the route, where people had staked out spots for hours before the celebrations.
Mayor Nancy Rotering said the violence had "shaken us to our core" and the state Governor JB Pritzker said it was a "uniquely American plague" of gun violence.
What's happening with the hunt for the gunman?
At about 10am AEST, police had Robert E Crimo III in custody.
The 22-year-old, who was named hours earlier as a person of interest, was pulled over by police after a brief pursuit.
Officials said a rifle had been recovered from the scene.
What do we know about the victims?
At least one of those killed was a Mexican national, a senior Mexican Foreign Ministry official said on Twitter.
Sheriff's department spokesperson Christopher Covelli said at a news conference that several of the victims died at the scene and one died after being taken to hospital.
At least 30 people, including four or five children, were injured in the shooting — most with gunshot wounds.
A hospital spokesman said those hurt ranged in age from eight to 85.
What did people see?
Witness Amarani Garcia, who was at the parade with her young daughter, told local news she heard gunfire nearby, then a pause during what she suspected was reloading, and then more shots.
There were "people screaming and running. It was just really traumatising", Ms Garcia said.
Social media video showed members of a parade marching band suddenly breaking formation and running away, and other images of people leaving their belongings behind as they sought safety.
"It sounded like a string of about 20 firecrackers that were let off inside of your metal garbage bin, it was that loud," Jeff Leon, 57, told CNN, estimating the gunfire lasted less than a minute.
Retired doctor Richard Kaufman, who was standing across the street from where the gunman opened fire, said he heard about 200 shots.
"It was pandemonium. A stampede. Babies were flying in the air. People were diving for cover," he said.
What happened in Philadelphia?
Gunfire broke out near the Philadelphia Museum of Art on the final day of a festival, with a large number of people watching a fireworks show after a concert.
Video from the incident showed hundreds of people running after the shots were fired, striking two police officers.
Both officers in Philadelphia were released from hospital after suffering non-life threatening injuries, the city's Mayor, Jim Kenney, said.
One officer sustained a graze wound to the head and the other a gunshot wound to the right shoulder, according to local television station NBC10.
Police were searching for the shooter.
What has the reaction been?
Frustration at another mass shooting in the nation has filtered through already.
This year, a school shooting in Texas saw 19 children and two teachers die, while 10 people were killed at a supermarket shooting in New York state, among other mass shootings.
Mr Covelli said it was a "very random, very intentional and a very sad day".
President Joe Biden said he and first lady Jill Biden were "shocked by the senseless gun violence that has yet again brought grief to an American community on this Independence Day."
Illinois Governor JB Pritzker said the mass shooting was "devastating" and called the gun violence a "uniquely American plague".
"Grief will not bring the victims back and prayers alone will not put a stop to the terror of rampant gun violence in our country," Mr Pritzker said.
"I will stand firm with Illinoisans and Americans: We must — and we will — end this plague of gun violence."
Mr Kenney said his administration was "pleading" with legislators to stop the "flow of guns into our city".
"Our traditions cannot and will not be ruined by the scourge of gun violence," he added.
ABC/wires
