Metro Manila (CNN Philippines, May 11) — Former and current high-ranking officials are responsible for death threats received by a businesswoman charged in a multi-billion-peso pork barrel scam, a lawyer said.

"These are the public officials who want Mrs. Napoles to keep silent and not expose her knowledge on the issues she was in. Mrs. Napoles informed us that she is still receiving death threats, " said a letter dated May 5, 2017 from lawyer Stephen David to the Department of Justice concerning his client Janet Lim Napoles.

The letter was received by the Department of Justice on Tuesday, May 9.

In it, David requested Napoles's transfer from the Correctional Institution for Women to the National Bureau of Investigation.

David is set to meet with Justice Secretary Vitaliano Aguirre on Thursday concerning the request.

David, however, said he lacked proof on the persons threatening to kill Napoles.

"The undersigned firmly believes that the threats, although no direct proof can be shown as to who the perpetrators were, come from former and current high-ranking public officials," he said.

On May 5, the Court of Appeals upheld a decision acquitting Napoles of the charges of serious illegal detention.

Related: Janet Lim Napoles acquitted in serious illegal detention case

The threats apparently worsened after news circulated that she may be tapped as a state witness in the possible reopening of investigation into the alleged pork barrel scam or Priority Development Assistance Fund (PDAF).

"There were several instances which her belongings were scattered inside her cell as if looking for something and almost every night, she started feeling unease for she felt someone is intently watching her," David said in the letter.