IS jihadists battle for control in northeast Nigeria
Today, this once-dominant figure seems to be leaving the scene after being badly wounded - or perhaps killed - last week in clashes with rival IS-allied jihadists.
If so, a major shift in Nigeria's 12-year-old jihadist insurgency appears to be developing as his enemies consolidate their grip, analysts say.
IS West Africa Province (ISWAP) is now targeting Boko Haram's soldiers after raiding Shekau's stronghold in Sambisa forest in Borno state, local and security sources said.
ISWAP has intensified attacks on Boko Haram factions, appointed its own chief in Shekau's enclave and executed 10 captured commanders who had refused to surrender, the sources said.
As ISWAP absorbs Shekau's fighters and territory, Nigeria's army potentially faces a more unified jihadist force, analysts say.
But ISWAP may also struggle to control or persuade Boko Haram factions loyal to Shekau outside Sambisa, especially in border areas.
"It may not be over yet, ISWAP will have to subdue or convince these camps to coalesce (them) into its fold to fully consolidate its control," said one security source.
Shekau was badly wounded after he shot himself last week to avoid capture by ISWAP fighters as they invaded his forest enclave, according to intelligence sources.
Two local Nigerian security sources said Shekau's men evacuated him badly injured, while local media reported he had died from his wounds, though his situation remains unclear.
Neither ISWAP or Boko Haram have released statements and the Nigerian military said only that it is investigating.
But whatever his status, fighting between rival factions has intensified.
On Wednesday, ISWAP jihadists in speedboats raided Boko Haram camps in Bosso in neighbouring Niger, leading to a battle with huge casualties, intelligence sources said.
"There was heavy fighting in Agadira, Lelewa and Kwatar Bauna between ISWAP and Boko Haram fighters which was bloody," said one intelligence source who asked not to be identified.
The two groups are also fighting in Mandara Mountains along Cameroon's border, where Boko Haram fighters are holding out, according to local intelligence sources.
Taking over Sambisa
During 12 years, Nigeria's military has struggled to end a jihadist insurgency that has killed more than 40,000 people and displaced around two million in the northeast.
Nigeria's two jihadist factions never resolved their differences -- mostly over Shekau's indiscriminate targeting of Muslim civilians and use of children suicide bombers.
Even before the Sambisa battle, ISWAP emerged as the more dominant force, carrying out large-scale attacks against the Nigerian military.
According to security sources in the Lake Chad region, ISWAP appointed Abu Mus'ab Al-Barnawi as its commander in Sambisa to replace Shekau.
Al-Barnawi is the son of Boko Haram founder Mohammed Yusuf and was chosen to replace Shekau after Boko Haram paid allegiance to IS.
Shekau rejected the change, prompting Al-Barnawi along with a bulk of fighters to split and form ISWAP in 2016.
Following its takeover of Sambisa, ISWAP sent messages to locals in the Lake Chad region, telling them they were welcome to its self-declared "caliphate", said Sallau Arzika, a fisherman from Baga.
Locals were chased out of the lake islands after ISWAP accused them of spying for the military. Al-Barnawi said they could now return for fishing and trading after paying tax, with the assurance they would not be harmed, Arzika said.
ISWAP told locals they declared a ceasefire on Nigerian troops to focus for now on fighting "tyrant" Boko Haram until they submit to its leadership or are killed, he said.
Al-Barnawi rounded up 30 Boko Haram commanders including two of five being considered to replace Shekau, according to intelligence sources and locals with knowledge of jihadist activities.
Maiduguri at risk?
Jihadist infighting may give Nigeria's army opportunities to take advantage.
But should ISWAP absorb part of Shekau's men, jihadists gain resources to cut off roads around Borno state capital Maiduguri and further test troops already heavily reliant on air power, Peccavi Consulting, a risk group specialising in Africa, said in a note.
"If ISWAP convinces Shekau's forces to join them, they will be controlling the majority of the enemy forces as well as having a presence in most of the ungoverned spaces in the North East," it said.
Since 2019, Nigeria's army has pulled out of villages and smaller bases to bunker down in so-called "supercamps", a strategy critics say allows jihadists free roam in rural areas.
Taking Shekau's enclave strengthens ISWAP, but expanding too quickly may also prompt a major military response like in 2015 when Chad troops crossed the border to help oust Boko Haram.
"ISWAP may be smart enough not to expand too far, too fast," Alexander Thurston, assistant professor of political science at the University of Cincinnati, wrote in the Lawfare blog.
"As long as it does not significantly upset this status quo, ISWAP retains influence over millions of lives, the ability to extract taxes from some rural populations, and a considerable amount of autonomy and freedom of movement."