This file photo taken on June 28, 2015 shows French police escorting Yassin Salhi.

A man jailed in France for beheading his boss and trying to blow up a chemicals plant in June has committed suicide, a prison official said on Wednesday.

Yassin Salhi, who French authorities said had links to Islamist militants, had denied that the attack was politically motivated, saying he had had an argument with his boss.

The 35-year-old delivery man hanged himself in his cell, prison union official Marcel Duredon told journalists.

Salhi was arrested in June at a site belonging to U.S.-based gas and chemicals company Air Products, near the severed head of his boss and next to flags bearing declarations of adherence to the Islamic faith.

The incident took place on the same day as deadly militant attacks at a Tunisian beach resort and a Mosque in Kuwait, and while France was on high alert following January’s Charlie Hebdo killings.

Examination of one of Salhi’s phones showed he had taken a picture of himself with the severed head before his arrest and sent the image to a number belonging to a French national last traced to the Islamic State of Iraq and Syria (ISIS) stronghold of Raqqa in Syria.

French authorities say Salhi associated with Islamist militants and was identified between 2006 and 2008 as being at risk of becoming radicalized. But he had a clean criminal record and did not show signs of planning any attacks.

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