The inquiry into the 2017 Manchester Arena bombing learnt that Ismail Abedi, the elder brother of Salman and Hashem Abedi, posted a picture on Facebook indicating that he sympathised with ISIS.
The image shows Salman Abedi pointing his left index finger skywards outside a mosque.
Detective Chief Superintendent Simon Barraclough told the inquiry that the gesture has come to mean allegiance to ISIS.
It was not the only troubling image found on Ismail Abedi's Facebook account.
He is seen holding an array of weaponry, including machine guns and rocket launchers, alongside other males in camouflage also holding weapons.
Other posts included shots of decapitated and dismembered bodies – and even of someone being burnt alive.
Mr Abedi is expected to give evidence to the inquiry into the May 2017 Manchester Arena attack that killed 22 people attending an Ariana Grande concert.
Although he is not considered a suspect, the interrogation will be uncomfortable for Ismail Abedi after the inquiry heard on Monday that his fingerprints were found on the car used to store the explosives.
His younger brother Hashem Abedi is serving a 55-year life sentence for helping Salman Abedi to build the bomb. The National reported on Monday that he had confessed to playing a full part in the attack after years of denying any involvement.
It is not only the Abedi siblings under the microscope in the inquiry. Their parents Ramadan Abedi and Samia Tabbal also face scrutiny. Detectives wish to speak to them on suspicion of terrorist offences linked to the forensics and financing of the attack.
"There are pieces of evidence that we need to talk to Ramadan about, some of that is forensic and some of that is about his account about coming to the UK to collect Hashem and Salman and returning with them – and some of it is his mindset,” said DCS Barraclough on Tuesday.
The forensic evidence relates to Ramadan Abedi's fingerprints, which were also found on the car that carried the offending explosives.
Ms Tabbal is wanted for questioning on suspicion of financing terrorism, DCS Barraclough said. Welfare benefits had continued to be claimed in her name and then used by her sons, despite her living in Libya.
The Manchester Arena inquiry was established to examine the circumstances of the attack and to determine whether any opportunities to prevent it were missed.
Last month, it was told by a top counter-terrorism official that a new law to improve safety and security at public venues and spaces in the UK will be groundbreaking.
The inquiry is expected to run until the spring of 2021.