In other Facebook posts, he appeared to support the Iranian missile attack against Israel in June and shared a video of a Palestine protest in Manchester.
Jihad Al-Shamie is understood to have entered the UK as a young child and was granted British citizenship in 2006 when he was around the age of 16.
Following the attack, his father wrote on Facebook saying his family “distanced itself” from the incident.
“The news from Manchester regarding the terrorist attack targeting a Jewish synagogue has been a profound shock to us. The Al-Shamie family in the UK and abroad strongly condemns this heinous act, which targeted peaceful, innocent civilians,” he wrote.
“We fully distance ourselves from this attack and express our deep shock and sorrow over what has happened. Our hearts and thoughts are with the victims and their families, and we pray for their strength and comfort.”
Faraj Al-Shamie was contacted for comment.