Lawyer Nizar Saghieh of the Legal Agenda group told Reuters that Bitar should stop prosecuting Diab as soon as he was notified of the case, adding that Bitar could sue others in the case.
Diab was charged over the port explosion on August 4, 2020, in which more than 215 people were killed.
Diab did not attend at least two interrogation sessions set by Bitar, who almost all the senior officials he sought to question abstained from appearing before him.
A lawyer representing Hassan Diab did not immediately respond to a request for comment.
Along with a number of former ministers accused by Bitar in the case, Diab said the judge does not have the power to prosecute him.
They filed a series of lawsuits, requesting that Bitar be removed from the investigation.
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