U.S. Attorney William McSwain said on Friday his office is investigating after a woman was hit in the face during Mass at the Cathedral Basilica of Saints Peter and Paul.
McSwain said the attack is "potentially a federal crime."
"No decision has yet been made about potential charging," McSwain said.
Philadelphia police said earlier this week that a suspect was identified and contacted, and based on that contact charges were deferred. No further details were released.
The Sunday morning Mass was streaming online when the attack happened.
Video shows Sarah Contrucci, 40, a longtime lector, leaving the altar as a woman punches her twice. It's still unclear what sparked the attack.
Contrucci told Action News in an interview on Monday that the woman muttered something to her before she was hit.
"Something about respecting God and I couldn't make out much else," said Contrucci. "I was worried she was going to attempt to make a scene of some sort to disrupt the Mass, so I was thinking I could do everything to keep the situation calm and respectful...I just wanted to get back to my pew and allow the Mass to resume."
Mass was able to resume and the suspect walked out of the church.
Contrucci did not require medical attention.