First responders take away victims from the scene of a mass shooting at a July 4 parade in Chicago, Illinois, United States, July 4, 2022. /CFP

A "person of interest" has been arrested after a mass shooting at a July 4 parade in Highland Park, U.S. state of Illinois, which left six people dead, local police said on Monday.

The arrest follows an hours-long manhunt. Police are still describing the 22-year-old man in custody, Robert E Crimo III, as a "person of interest" while they investigate and connect him to the scene.

Among the deaths, five people died at the scene and one individual died at the hospital after suffering severe injuries, according to the police.

A total of 26 patients were received at Highland Park Hospital, reported CNN. The patients ranged in age from 8 years old to 85 years old, according to Brigham Temple, medical director of NorthShore University Health System.

Highland Park is located about 25 miles north of downtown Chicago. The shooting began at 10:14 a.m., when the parade was approximately three-quarters of the way through.

Security forces are deployed to the scene after multiple people were reported to have been shot at a July 4 parade in Chicago, Illinois, United States, July 4, 2022. /CFP

Witnesses said parade attendees initially thought gunshots were fireworks, reported CNN. They realized a shooting happened after seeing a man with a wound on his head, the reports said.

U.S. President Joe Biden responded on Monday to the mass shooting, saying he and first lady Jill Biden are "shocked by the senseless gun violence that has yet again brought grief to an American community on this Independence Day."

He said he's surged federal law enforcement to "assist in the urgent search of the shooter."

In a speech, Illinois Governor J.B. Pritzker said "there are no words" to describe the loss and grief the families of those killed and injured feel this holiday.

"It is devastating that a celebration of America was ripped apart by our uniquely American plague," Pritzker said.

It is the latest mass shooting to hit the U.S. Halfway through the year, there have been at least 309 mass shootings, including at least three others on July 4, though without any fatalities, according to the Gun Violence Archive.

On June 25, Biden signed the first significant federal bill on gun safety in decades, just days after the Supreme Court ruled that Americans have a fundamental right to carry a handgun in public.