The alley they searched is situated among a mix of two-story rowhouses, some occupied by longtime residents, others vacant.
Maria Sierra, 63, said she was making coffee about 6 a.m. when authorities began pounding on her neighbor's door, yelling, "Open it, open it!"
"It is really scary," she said later in the day, as FBI members - some wearing T-shirts that read "Technical Hazards Response Unit" - canvassed the area and walked through tents behind her apartment.
She and another resident said that a man recently had moved into the unit authorities were searching and that he had lived there for about seven months. His identity was not known.
Sierra said the neighborhood was "nasty" for some time - filled with gunshots, fights, and drug lords - but that it had improved in the last year.
In November, federal and local authorities busted a major drug network allegedly operating in the area, near Fourth and York Streets. The network, which had ties to Mexican cartels, sold up to $50,000 a day in heroin and cocaine, authorities said.
David Torres, 29, who grew up in the neighborhood, reflected Tuesday on some of the past turmoil, saying he was "born and raised" hearing gunshots and other crime. But "for FBI being around here," he said, "that's something new."
Staff writers Jeremy Roebuck and Julia Terruso contributed to this article.