BEIRUT, Lebanon — When the youngest son of the former Libyan leader, Col. Muammar el-Qaddafi, was arrested in Lebanon last week in connection with the unsolved disappearance of Moussa al-Sadr, an exalted Lebanese Shiite cleric who vanished while visiting Libya in 1978, speculation sprouted about new information concerning one of the biggest whodunits in the treacherous politics of the Middle East.

On Monday, the mystery deepened with news that the son, Hannibal Qaddafi, may have been forcibly — and illegally — brought to Lebanon against his will in a plot involving the son of a colleague of Mr. Sadr’s, Sheikh Mohammad Yacoub, who disappeared along with Mr. Sadr and a third companion in Libya nearly four decades ago.

Lebanese officials said that Sheikh Yacoub’s son, Hassan Yacoub, a former member of Parliament, had been formally placed under arrest on suspicion that he had helped orchestrate the abduction of Hannibal Qaddafi from Damascus, Syria, in the days preceding Mr. Qaddafi’s arrest here. The officials and a lawyer for Mr. Qaddafi said he had been living in Syria, granted asylum by the Syrian government in the aftermath of Colonel Qaddafi’s violent fall from power in October 2011.

Even with the arrest of Mr. Yacoub, Hannibal Qaddafi remains under arrest in Lebanon, accused by an investigative magistrate of not providing all information he may know about the disappearance of Mr. Sadr, Sheikh Yacoub and Abbas Badreddine, a journalist, while they were visiting Libya at Colonel Qaddafi’s invitation in August 1978. It is unclear what information Hannibal Qaddafi, 40, could possibly share, since he was a small boy at the time.

The disappearance of Mr. Sadr and his colleagues in Libya remains a potent mystery in Lebanon, where Mr. Sadr is revered as a hero to poor Shiites from the tumultuous days of the 1970s, when Lebanon was convulsed by civil war, a spillover of the Israeli-Palestinian conflict and other problems. The disappearance has been the subject of numerous criminal inquiries. Colonel Qaddafi, a notoriously erratic and unpredictable dictator, insisted that he had nothing to do with it and that the Lebanese visitors vanished after having flown to Italy.

Many Lebanese say they believe that three Qaddafi aides, disguised as the Lebanese visitors, flew to Italy with their luggage to create a false narrative about where they had last been seen.

Mr. Qaddafi’s lawyer, Boshra Khalil, said in a telephone interview that her client had been beaten and thrown into a car trunk when kidnapped from Syria by people she described as bodyguards of Mr. Yacoub.

The Lebanese news media have widely reported that Mr. Qaddafi had been brought to Lebanon in Mr. Yacoub’s car. His abductors forced Mr. Qaddafi to read a statement broadcast on Lebanese television on Dec. 10, in which he said that they were disciples of Mr. Sadr and that their cause was just. They turned him over to Lebanon’s Internal Security Forces the next day, and he was placed under formal arrest on Dec. 14.

Ms. Khalil said she expected him to be released soon. “He is not guilty, and he was 3 years old when Imam Sadr went missing,” she said. “He knows nothing about the case.”

Hwaida Saad reported from Beirut, and Rick Gladstone from New York.

By HWAIDA SAAD and RICK GLADSTONEDEC. 21, 2015

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