Prosecutors could seek an indictment against David Roland Waters, 53, a former office manager for Ms. O'Hair, by next month, the Austin American-Statesman reported Thursday.
Mr. Waters was convicted of stealing from atheist organizations led by Ms. O'Hair. Mr. Waters, who is serving 60 years in prison, also faces time in federal prison on weapons convictions.
Daryl Fields, spokesman for the U.S. attorney's office in San Antonio, would neither confirm nor deny the report.
"We have nothing to say other than the investigation is continuing and is active," Mr. Fields said.
Calls to Mr. Waters' lawyers were not immediately returned.
Last month, Gary Paul Karr was sentenced to life in prison for extorting money from Ms. O'Hair, her son Jon Garth Murray and granddaughter Robin Murray O'Hair, and for other crimes.
The trio has been missing since 1995. They were last seen in San Antonio.
Prosecutors contend they were kidnapped, murdered and dismembered. No bodies have been found.