Kenyan police have named homegrown militant Islamist Mohamed Mohamud, a quietly spoken former teacher, as the alleged mastermind of the massacre of 148 people in a university in Garissa.
Known also by the alias Kuno, as well as 'Dulyadin' and 'Gamadhere' - meaning "long armed" and "ambidextrous" - the alleged Shebab member is also wanted in connection with a string of recent cross-border killings and massacres in Kenya's northeastern border region.
Police have offered a 20 million shilling (S$290,000) bounty for information leading to his capture.
Mohamud is a Kenyan national and an ethnic Somali - like more than two million other Kenyans or some six per cent of the population. The minority mainly lives in the country's vast, impoverished and arid northeast, where Garissa is one of the largest towns.
Kenya's ethnic Somali region is also claimed by the Shehab as part of Somalia itself, and has long been lawless, including the brutal secessionist 1963-1967 "Shifta war".
While Mohamud did not take part physically in the Garissa attack, students who survived the massacre described the attackers as men like him: speaking Kenya's Swahili language well, with some suggesting they may have been Kenyan too.
The poster of Mohamud shows a slender man with a short beard. Kenyan police sources say he used to teach in a madrassa in Garissa, but later became radicalised and crossed the porous border into southern Somalia to join the Islamic Courts Union, a precursor to the Shebab.
An AFP correspondent who met him in the Somali capital Mogadishu in 2008 and 2009, when the majority of the city was under Shebab control, said Mohamud was a well-known and feared commander.
He commanded a much feared Islamist unit in Mogadishu called the "Jugta-Culus" - or "heavy strikers", who carried out some of the toughest fighting. Mohamud, however, also appeared in person as educated as well as "quiet and gentle".
He appeared in several propaganda films showing Shebab battles in southern Somalia, and later was a commander in the southern Somali Ras Kamboni militia, under the warlord Ahmed Madobe, a former Islamist commander turned Kenyan ally.
In the murky world of Somali armed groups, politics and clan loyalties, Madobe's forces helped Kenyan forces seize the key port of Kismayo in 2012.
While Mohamud is on the run, Madobe now leads southern Somalia's Jubaland region. But under pressure on their home soil, the Shebab have reached into Kenya to carry out attacks and find recruits among disaffected youth in the Muslim-majority coastal and northeast regions.
In November, the Al-Qaeda-affiliated group claimed responsibility for holding up a bus outside Mandera, separating passengers according to religion and murdering 28 non-Muslims. Ten days later, 36 non-Muslim quarry workers were also massacred in the area.
A Shebab statement on Friday warning Kenyans of further bloodshed, said the gunmen carried out the Garissa attack in revenge for the "systematic persecution of the Muslims in Kenya".
Attacks cited include Kenya's 1984 Wagalla massacre, when Kenyan troops trying to put down local conflict killed an unknown number of people - officially less than a hundred, while others claims up to 5,000 people.
Cash rewards for other Shebab commanders - offered by the US, and unlike Mohamud's bounty, in their millions of dollars - are believed to have led to information that have resulted in a series of air strikes in Somalia to assassinate them.