Two officers, John Meehan, 44, and David Schiazza, 54, raced to a home on Lewis Avenue, the scene of the reported shooting. When they arrived, they began talking to someone at the property.
Moments later, gunfire erupted from the house.
Mr Meehan was hit in the left arm, and Mr Schiazza took a bullet to the leg.
The gunfire set off a chain of events that would end with the home burned to the ground, and all six people inside — including children — dead.
"This is going to take awhile. Okay? We're going to be very safe," Delaware County District Attorney Jack Stollsteimer told NBC10 on Thursday morning.
The shooting was over, and the flames licking from inside the house were subdued, but dangers to responding officials — largely due to the structural integrity of the scorched home — lingered.
"This is a recovery operation," he said. "There's nobody alive inside that burned out hulk of a house."
Police haven't yet confirmed the identities of anyone killed inside the house. At least one of the people killed was a child; the William Penn School District confirmed as much when it sent a letter to parents noting the tragedy.
"Like all of you, we are at a loss regarding the horrific tragedy that happened in East Lansdowne yesterday and how that has affected our community. We are still awaiting additional information as this is an ongoing police investigation," a district spokesperson wrote in the letter, first obtained by NBC10.
The letter said counselors would be available for anyone needing mental health services in the wake of the incident, and encouraged parents and students to reach out if they needed help.
During a press conference on Wednesday evening, Mr Stollsteimer said that between six and eight people lived in the home, and were unaccounted for at the time of the presser. He confirmed that number included children.
“We’re aware that there are at least six to eight people who are unaccounted for from that family. It is our terrible fear that they may have been inside that house when it was burned," he said at the briefing. "We are hopeful that that is not true. But we will not know until [Thursday] morning.
Mr Stollsteimer said he expected the recovery work to take "hours" or possibly "days" due to the dangers presented by the burned out house.
“Unfortunately, I’m very confident that we’re going to find at least one body in there, and I think we’re going to find many more, several more,” he told local broadcaster KYW-TV. “It is going to be a tragic story."
Whether or not an 11-year-old was actually shot has still not been verified by law enforcement.
After the initial hail of gunfire, police officers from Upper Darby heard the call that two officers from East Lansdowne had been shot. Using riot shields, they managed to safely drag the injured officers out of harm's way.
Both Mr Schiazza and Mr Meehan are expected to recover from their injuries.
While the injured officers were driven to local hospitals, the chaos at the house continued. Gunshots were still coming from inside the house when officials realized that, somehow, the home had caught fire.
Officials believe that the fire was intentionally set by someone inside the house, but further investigation will be necessary to confirm the exact cause of the blaze.
The fire department rushed to the scene, but firefighters could only wait and watch as the flames intensified; someone was still shooting from inside the house, making it impossible for them to start battling the blaze.
Once the gunfire had died down, firefighters cautiously moved closer and began dousing the house to stop the flames. Police were set up with rifles trained on the house in the event the gunman was still alive and attempted to shoot at the firefighters, Mr Stollsteimer said during a televised interview.
By 6pm on Wednesday evening the gunfire had stopped, and the fire was mostly under control at the home. Mr Stollsteimer said he would not allow anyone into the building until he knew the shooter was incapacitated and the home was safe.
"We're not going to make entry into that house until we know that the fire is under control and that it's safe for those officers to go in there," he said during a Wednesday evening presser. "We don't want another single officer hurt tonight in Delaware County."
He said there was "like six feet of water in the basement" of the house, and warned that the "walls could collapse at anytime."
Excavators were called in to help begin peeling away portions of the ruined home to assist in the recovery.