He hasn't been seen for over a year, remaining in the shadows while Bashar al-Assad has been the public face of Syria.

But Maher al-Assad has in many ways played a more decisive role in the country's civil war than his elder brother, commanding its most formidable military division as it claws back losses and leading the defence of Damascus against an opposition that remains entrenched on the capital's outskirts. The question many Syrians are asking, after last week's revelations of an apparent chemical attack on civilians in rebel-held areas, is what role the president's brother may have played in the atrocity.

Maher has remained a senior member of the Ba'ath party's central committee and a central pillar of a police state that, despite the ravages of war and insurrection, remains one of the most effective in the world.

As the trajectory of Syria's war has wobbled throughout the past year, opposition gains in parts being offset by regime advances elsewhere, the 4th Armoured Division Maher commands has been a chief protagonist on behalf of the regime.He has acted as division commander since at least 2000, and at the same time leads Syria's other premier fighting force, the Republican Guards. Both units have been at the vanguard of the war since its earliest days, and were active again last week as loyalist forces launched their biggest operation yet to root out rebel groups from the capital.

It was while this operation was under way that thousands of residents of east Ghouta were exposed to what scientists increasingly believe was a nerve agent, possibly sarin. Attempts to pin down who was responsible for the attack are now the subject of a global intelligence effort that has already started to zero in on loyalist military units as the likely suspects.

In the days since, Syria has persistently denied having used its stocks of sarin to shell the area.

The 4th division has remained relatively unaffected by desertions and defections that plagued other divisions in the first 18 months of the war. Until about then — 18 July last year — Maher was visibly in charge. Frontline troops saw him often, especially in the hotspots of what was then more an insurrection than the full-blown civil and proxy regional war it is now.

In Deraa he personally led a siege by 4th division troops in March 2011, in response to a spark of defiance by a group of schoolboys, who wrote on a mosque wall calling for Bashar to leave. The division left an unambiguous calling card.

"He came to see us one weekend down there," said a former 4th division conscript who fled to Istanbul soon afterwards. "He told us not to shoot at the men with guns, because they were with us. He told us only to shoot at people without guns, that they were the terrorists.

"It took me a while to protest at that. He made us shoot at their hearts and heads. And anyone that was shooting high and wide [deliberately] would be beaten, or killed."

In Jordan's Zaatari refugee camp, only nine kilometres south of Deraa, which is now home to a sizeable chunk of the city's residents, Maher's name is spoken of with visceral anger.

"His brother is a puppet for Maher and the Iranians," said Khaled Othman, a plumber from the city, standing in the flap of a UN supply tent. "Maher is the devil. He personally tried to annihilate us just because we defied him. He took pleasure in it, along with his closest officers. Did you see the video of him in the prison?"

The refrain is commonly asked in communities that support the Syrian opposition. It refers to a prison revolt in 2008 that Maher was asked to put down; a task he carried out with brutal efficiency, killing many who had taken guards and soldiers hostage, then filming the bodies with his camera phone.

The phone video is often showed by supporters of the Assad regime as purported evidence of the strength of the brothers. But between the statesman and the general, it has been Maher who has inspired more fear — and speculation.

His last appearance in public was several weeks before an explosion in a meeting room in central Damascus killed security chief, and the Assads' brother-in-law, Assef Shawkat, who had married their sister, Bushra. Also killed that day was the defence minister and several other members of the inner sanctum.

Rumours have circulated since that Maher was also in the room at the time and was wounded. Last year Abdullah Omar, a former press officer in the presidential palace who defected in September, said he had seen Maher visit the palace and he had appeared to have lost part of a hand and leg.

The suggestion has not since been confirmed. Turkish officials believe that the younger Assad was wounded that day — one of the few times that the opposition has got so close to the seat of power. "But he is alive and functioning," said one senior Turkish diplomat. "And the 4th division is still one of their better units."

In Lebanon, where in more settled times leaders from all sides of politics beat a regular path to Damascus, there has been nothing from Maher for more than a year.

"We know his wife is in Dubai along with Bushra," said the leader of one political bloc. "And we know that Bashar will have a hard time keeping him in his box. If they think they are winning, they will behave without any restraint. And if they did the chemical attack, he won't be far away from it."