The law was recently expanded to include victims with disabilities, and the FBI is taking a broad look at the complex, multi-state case, said the bureau official, who was not authorized to discuss the case publicly.
A fourth person was arrested Wednesday on suspicion of kidnapping as part of the alleged scheme. Jean McIntosh, 32, is the daughter of alleged ringleader Linda Weston. A landlord described McIntosh as a former Army nurse who lived in an apartment above the basement with her two teenage children.
McIntosh was arrested a day after Philadelphia police took 10 young people linked to the case into protective custody.
The six juveniles and four young adults found Tuesday, ages 2 to 19, are thought to be related to the suspects and perhaps some of the victims, police spokesman Lt. Raymond Evers said. Authorities are conducting DNA tests and obtaining birth certificates to try to determine the nature of the various relationships.
Police described the 19-year-old as Weston's niece and say she was found malnourished and showed signs of abuse.
McIntosh was arrested around 3:45 a.m. after detectives questioned her about the case. Weston had arrived at her apartment building from Florida this month with two men, the four disabled adults and others in tow, according to neighbors, the landlord and police.
McIntosh is expected to be arraigned later Wednesday on kidnapping, conspiracy and other charges, District Attorney Seth Williams said. It's not immediately clear if she has an attorney. According to her landlord, she had a key to the basement.
Weston's defense lawyer has not returned calls seeking comment.
Authorities in at least two states missed opportunities to help the disabled adults, who were found in a locked boiler room by the landlord Saturday. Police believe Weston had been stealing their Social Security disability checks, perhaps as part of a much larger fraud scheme. They found dozens of other Social Security and identification cards, along with power of attorney documents, in a search of McIntosh's apartment, where Weston had been staying.
Weston was legally disqualified from cashing the victims' government disability checks because of her criminal past.
But she apparently did anyway, enabled in part by a lack of accountability and follow-through by government agencies and police in Philadelphia and West Palm Beach, Fla.
Weston remains jailed on $2.5 million bail, along with Gregory Thomas, 47, whom Weston described as her boyfriend, and Eddie "the Rev. Ed" Wright, 50. They face similar charges.
Landlord Turgut Gozleveli discovered the victims after he heard dogs barking in the basement. The door to the basement room was chained shut, but Gozleveli got inside and lifted a pile of blankets to find several sets of eyes staring back at him. One man was chained to the boiler.
Police identified the victims as Derwin McLemire, 41, of North Carolina; Herbert Knowles, 40, of Virginia; and Tamara Breeden, 29, and Edwin Sanabria, 31, both of Philadelphia.
Knowles was reported missing in Norfolk, Va., in December 2008. According to a report by Norfolk police, Knowles' mental health case worker reported him missing when she couldn't reach him and family members failed to hear from him.
The case worker, who did not return a call from The Associated Press, reported that Knowles' Social Security checks were going to a Philadelphia address. The report said Philadelphia police went by the address and were told no one there had ever heard of Knowles.
A Philadelphia police report shows that officers knocked on the door on Dec. 5, 2008, and the woman who answered said that no one by the name of Herbert Knowles lived there. The report showed no sign of a follow-up or any indication that the responding officers had any reason not to believe the woman who answered the door.
Norfolk police spokesman Chris Amos said authorities did not continue looking for Knowles because, as an adult, he was under no obligation to report to the case worker.
"It's not illegal to be missing," Amos said. "A lot of people are missing by choice."
Police in West Palm Beach, where Weston lived earlier this year with the four mentally disabled adults, also missed a chance to crack the case.
Chase Scott, a spokesman for West Palm Beach police, said officers were sent to the house several times for complaints about trash and code violations.
Investigators said they're trying to piece together details of Weston's scheme, including how long it went on, how much money it brought in and how many people in all were victimized.
Weston had been convicted in the starvation death of a man nearly 30 years ago, though it's unclear how much prison time she served.
The Social Security Protection Act of 2004 generally bars people who have been imprisoned for more than a year from becoming representative payees, those who cash someone else's check. Yet a 2010 report by Social Security's watchdog found that staff members do not perform background checks to determine if payees have criminal records.
The report from the Social Security Administration's Office of the Inspector General said that people who apply to become payees are supposed to answer a question on whether they've ever been convicted of an offense and imprisoned for more than a year. But the report noted that the agency recognizes that self-reporting of such information "is not always reliable."
The inspector general said that in the cases it reviewed, about 6 percent of non-relative payees had been imprisoned for longer than a year and "may pose a risk to the beneficiaries they serve."
A Social Security spokesman declined to provide details of the agency's investigation into Weston but said the agency recently strengthened oversight of payees.
The 2009 Matthew Shepard and James Byrd Jr. Hate Crimes Prevention Act is named for two victims of notorious hate-based killings and expands earlier federal hate-crimes law to include sexual orientation or disability, among other things.
The law has been used sparingly since its passage. The first to go to trial was the case of Frankie Mayberry, of Green Forest, Ark., who was convicted in May of attacking a car last year with five Hispanic men inside it.