Prosecutors say the scam had been going for five years and that its key members were a police officer and a Sofia municipal official.
A total of eight people have been arrested over the allegations.
"The police officer's role was to research people who were either elderly or who had died recently and owned expensive property," a statement from anti-organised crime prosecutors on Friday said.
The victims were chosen on the basis of having no close relations who would be aware of their inheritance.
Once the victims had been identified, the municipal employee "deleted any remaining rightful inheritors from the records" while issuing fake backdated marriage certificates, the statement went on.
The scheme is thought to have amassed at least 15 properties, worth more than 500,000 euros ($554,000) in total.
"It's an unusual way to go about property fraud, the methods are not the traditional ones," said Dimitar Petrov, head of the Specialised Prosector's Office.
One of the fake marriage certificates in question was made to look as though it had been issued in Russia.
"It's more difficult to verify the authenticity of documents issued by a country which isn't in the European Union," explained interior ministry official Stefcho Bankov.
The alleged network was uncovered when two suspects posing as a woman and her husband tried to withdraw 600,000 leva (300,000 euros) from a bank in Sofia, ostensibly for the cash purchase of a flat.
The woman was in fact not married and recently deceased but her death had been removed from public records.