WASHINGTON—Federal investigators have charged 530 people for allegedly defrauding more than 73,000 desperate homeowners around the country who fell behind on mortgage payments, leaving them vulnerable to con artists offering to help them avoid foreclosure.

At a news conference discussing the results of a yearlong probe, Attorney General Eric Holder and Housing and Urban Development Secretary Shaun Donovan estimated the homeowners lost more than $1 billion in the fraud schemes.

According to court records in the 285 criminal cases brought in the investigation, the struggling homeowners would pay substantial fees with promises that so-called investors would purchase the victims' mortgages. Other homeowners paid hefty fees in exchange for what turned out to be false promises that more favorable mortgage terms would be negotiated on their behalf.