A federal grand jury in Manhattan is probing the alleged theft of eight centuries-old books from the New York Public Library that have been missing for three decades and are worth a fortune — including an original Benjamin Franklin manuscript and seven Bibles.

Manhattan US Attorney Preet Bharara empaneled the grand jury earlier this month at the NYPL’s request and has used subpoenas to temporarily take possession of the “priceless” books — including an 18th-century workbook from Franklin’s printing house, appraised at more than $1 million, and seven Bibles published between 1672 and 1861, according to court documents filed by prosecutors this week.

The feds are pursuing potential criminal charges in the bizarre case even though a state judge has been asked to rule on rightful ownership. Days after the federal probe began, Margaret Tanchuck of Glen Head, NY, filed a lawsuit in Nassau County Supreme Court against the NYPL for threatening her with criminal action — even though the books have been in her family for nearly 30 years and she says she only learned they existed while recently administering her late parents’ estates.

Daniel Arshack, a lawyer for the 50-year-old Tanchuck, said he believes the feds have overstepped their bounds by getting involved in a civil dispute that should be decided by a judge. He said his client has no way of knowing how her family came into possession of the books and that the NYPL has no documentation showing how the items left its possession.

“It’s just horrifying that a woman who is having to deal with the passing of her mother and father should suddenly find herself being the subject of a federal investigation based only on her doing exactly what the law requires her to do” by getting the books appraised, Arshack said.

The lawyer also said he is especially concerned because Bhahara’s office has not given him any assurances that it will return the books to his client — even if she prevails in her lawsuit. He has filed legal papers seeking to have the grand jury subpoenas quashed or modified, and a hearing is set to be held on the matter Tuesday in Manhattan federal court.

Manhattan auction house Doyle New York jump-started the controversy while appraising the books in July 2014 by determining that serial numbers on the items revealed they belonged to the NYPL.

The books were once reference-desk materials that couldn’t be checked out by the public, and although the NYPL had never reported them missing, it began vigorously pursuing their return after learning of their existence. A source familiar with the case said Tanchuck offered to sell the books to the NYPL for “millions of dollars,” but library officials opted to contact the feds rather than shell out plenty of Benjamins.

“This material was evidently stolen from the Library, and now someone is trying to profit from it,” said NYPL spokesman Ken Weine in a statement. “We will aggressively work to ensure that this material is returned to the public domain where it belongs.”