They believe he is between 16 and 20 years old.
Luanda Marinho, who lives in Center City, said she and a friend came face to face with the masked individual before their morning run on August 12.
"I just want to run in peace. It's not much to ask," Marinho said. "I was definitely scared because it's not normal for someone wearing a mask at that time of the day."
Marinho said the encounter happened at 12th and Chestnut streets.
"He's like wanna race me? But I'm faster than you guys. Let's race. He sounded very young."
Police say the same suspect harassed multiple people before and after 5:30 a.m. that morning, including a 40-year-old runner near City Hall.
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Philadelphia Police Captain Jason Smith said the suspect confronted the woman while holding up his phone.
"He's holding his cell phone up in front of her at which time he asks her, are you ready to die?"
Police say the suspect also entered the Dunkin' on Chestnut Street.
"He's refused service due to wearing this mask," Smith said.
A Dunkin' employee told Action News the masked male frightened her and tapped a customer on the shoulder before heading toward 15th Street and South Penn Square.
There, a woman identified as Jameka W. screamed at the suspect and kicked him in the chest.
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Woman recalls encounter with masked man terrorizing people in Center City
"I didn't want him to harm anyone else on my way, and because he just seemed so disoriented about what his goals actually were," she said. "I was like OK, well, let's see if you could fight and he couldn't."
Jiu-Jitsu instructor Brian Rago said maintaining awareness is the best form of self-defense.
"Always maintain a distance, create more distance. There is no incentive to fight. No matter how skilled you are, there is nothing good coming from a fight in the street."
Police say the suspect fled to Suburban Station after the incidents. They are asking anyone who encountered him or knows his whereabouts to contact authorities.
According to investigators, he will face several charges, including terroristic threats, harassment, and simple assault.