Chief Inspector Frank Vanore said Wednesday that the two .40 caliber bullets were fired from the same weapon and likely traveled from "quite a distance away."
Vanore said he does not believe the officers were the intended targets.
He said a .40 caliber bullet can travel unimpeded more than 2,200 yards - or more than a mile.
The injured officers were standing at the base of the steps of the Philadelphia Museum of Art, but not next to each other when the shots were fired.
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In a statement issued Wednesday morning, District Attorney Larry Krasner asked the public to be patient as the investigation "will be very challenging."
"In the meantime, please know this: Deliberately shooting at another person is a very serious crime. Celebratory gunfire on any occasion or gunfire not aimed at another individual that nonetheless harms others is a very serious crime; applicable charges may range from Reckless Endangerment to Aggravated Assault to Murder," Krasner said.
Bullets that injured officers on July 4 came from same weapon: Police
Around 9:45 p.m. Monday, the two officers were working security detail for the Wawa Welcome America Party on the Parkway when they were struck.
Montgomery County Sheriff's Deputy John Foster, who is assigned to the bomb squad task force, sustained a graze wound to the shoulder.
A law enforcement source says Deputy Foster thought someone had slapped him on the back. That is until another officer looked at him, saw the blood and told him he's been shot.
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They then looked over at Philadelphia Police Officer Sergio Diggs and saw blood coming down his face.
Diggs said it felt like he had been hit over the head, or an object had hit him.
The bullet that hit Diggs was most likely traveling in a downward direction and was found lodged in his hat.
Both were treated and released from the hospital and are at home recovering.
Foster told investigators what he remembers.
"He's about 20 feet away on the same sidewalk on the side of the building," recalled Vanore. "He sees Diggs and starts moving towards him and he feels what he believes is an object hit him in the right shoulder. He believes he's hit with an object. He keeps walking and he looks down and sees some blood on his shirt."
Both projectiles were recovered: one in Diggs' hat, the other was found by rented bicycles at the bottom of the Art Museum steps.
Vanore said there are so many variables they need to know to determine where the gunfire came from.
"Because the bullets were moving so slow and didn't have much momentum left, a .40 caliber round can travel unimpeded. You don't know what the wind was, other variables, wind density."
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Vanore said no one reported hearing a gunshot and no other bullets were found in the area.
Still, officers made the decision to evacuate the Parkway.
"We didn't know what we were dealing with, the officers didn't even know they were shot," said Deputy Commissioner Joel Dales.
City council members say its time to double down, calling for more police support, increasing drone and surveillance technology and supporting legal stop and frisk interactions.
This comes as the city looks ahead to the Made in America festival later this summer. Police said they are now reexamining how perimeters are set up around the Parkway.
Anyone with video from the fireworks or from the time of the shooting is asked to send it to police.
A reward issued earlier this week for information leading to the shooter has grown to $42,500. Anyone with information is asked to call 911 or 215-686-TIPS.
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