Thomas Gregory, 47, is one of three suspects accused of detaining four mentally disabled people.
Linda Ann Weston, 51, the alleged ringleader, was convicted more than 10 years ago on a murder charge in Philadelphia.
Eddie Wright, 49, was also arrested. All three are accused of stealing the victims' Social Security checks.
Philadelphia (CNN) -- At first, a Philadelphia landlord said, he thought one of his tenants was attempting to conceal a dog after finding a dog dish in the basement of his apartment building.
But on a return trip to the basement Saturday, Turgut Gozleveli found something much more sinister: four mentally disabled people held captive, including one man chained to a radiator.
"It was terrible," he said Sunday, adding the people were surrounded by human waste. "I don't know how long they were there."
Three people have been arrested and accused of holding the people captive and stealing their Social Security checks.
Linda Ann Weston, 51; Thomas Gregory, 47; and Eddie Wright, 49, face charges including criminal conspiracy, aggravated assault, kidnapping, criminal trespass, unlawful restraint, and false imprisonment, Philadelphia police said in a statement Sunday.
The alleged victims include a 29-year-old woman and three men, ages 31, 35 and 41, said Philadelphia police Lt. Ray Evers.
Weston, the alleged ringleader, was convicted more than 10 years ago on a murder charge in Philadelphia, Evers said. She served eight years in prison.
All seven -- the three suspects and four alleged victims -- had been traveling together, he said, but did not know for how long. Weston is believed to have been in McLean, Texas, with some or all of the alleged victims. They then traveled to Florida and Philadelphia, Evers said.
Gozleveli said he was contacted Thursday by the block captain of a neighborhood group regarding suspicious activity. He checked out the basement that day and found a few things out of place, but no people. On Friday, he found the dog dish. On Saturday, after hearing a dog barking, he went to the building's sub-basement and found a door chained shut. He found the people after removing the chains.
"Other than the barking dog there was no sound," he said. "... I said, 'What the hell are you doing here?' No answer, and I called 911."
"This was a total surprise," Gozleveli said. "I never expected to find people. I just expected to find a dog."
The people, he said, "acted like children."
The four were in a room roughly 10 feet by 15 feet, Gozleveli said, and looked malnourished and disheveled. They had a metal bucket and a jug of what appeared to be orange juice, but no food. They also had pillows and blankets, but didn't know where they were or where they had come from, he said.
Authorities believe the four were trapped in the tiny room for up to a week. Evers said they suffered from bed sores and "injuries that are very, very hard to describe."
The alleged victims appeared to be malnourished and were taken to a hospital. They were listed in stable condition Sunday, police said.
The FBI is also investigating.
The northeast Philadelphia building is a former movie theater converted to a seven-unit apartment building.
Police cars were still outside the building Sunday afternoon.
The four people found in the basement were brought out on stretchers, said Danyell Tisdale, the block captain who notified Gozleveli. She said she was concerned about some of the things she'd seen recently, including people being brought in from the back of an SUV with out-of-state plates.
"I hope I did enough," she said. "I called."
CNN's Ross Levitt contributed to this report.