ISLAMABAD (AP) — Pakistan’s top investigation agency has arrested an opposition leader, who is a close aide of convicted former Prime Minister Imran Khan, for exposing official secrets and harming state interests, according to case documents revealed Sunday.

It’s the latest development in a standoff between the administration of outgoing Prime Minister Shehbaz Sharif and his predecessor Khan, who was jailed earlier this month on graft charges.

Shah Mahmood Qureshi was arrested Saturday night at his Islamabad home by the Federal Investigation Agency on charges of exposing a letter last year to claim that Khan was ousted from power by the U.S.

The letter has not been made public but was apparently diplomatic correspondence between a Pakistani ambassador to Washington and the Ministry of Foreign Affairs in Islamabad.

Cricketing hero Khan was booted out of parliament in April 2022 in a no-confidence vote. He insists his removal was engineered by Washington, Sharif's government and the military. All three deny the allegation.

Case documents about Qureshi’s arrest said he, Khan and others are involved in the communication of information in a secret classified letter to unauthorized people by “twisting the facts to achieve their ulterior motives and personal gains.”

The documents allege they conspired to misuse the contents of the letter to accomplish their “nefarious designs.”

Nobody from the Federal Investigation Agency was immediately available for comment.

Qureshi was arrested hours after he held a press conference Saturday evening demanding a level playing field for Khan's party, alleging that thousands of supporters are behind bars. He said the party will challenge any possible delay to elections, due to be held later this year.

Khan, who is serving a three-year term in a high security prison in eastern Punjab province, has also been arrested in the case, known locally as Cypher. Qureshi is standing in for Khan as leader of the Pakistan Tehreek-e-Insaf party, or PTI.

Sharif accused Khan of exposing an official secret document in an incident last year when he waved a confidential diplomatic letter at a rally.

Khan has more than 150 legal cases against him, including several on charges of corruption, “terrorism” and inciting people to violence over protests in May that saw his followers attack government and military property across the country.

Authorities have moved to rein in the outspoken politician’s supporters and allies since May, detaining thousands of Khan’s followers for their alleged involvement in the turmoil.