But there has been no official boast of responsibility from the Al Qaeda-linked group, and Kenyan ministers and security officials have stopped short of directly blaming its fighters.
Instead, analysts suggested, the explosion, which injured 38 people, was more likely carried out by a “lone wolf” bomber sympathetic to Al Shabab, but operating outside of its direct command and control apparatus.
The blast ripped through an informal market a block to the east of Nairobi’s central business district, destroying stalls selling clothes, shoes, cellphones, and cheap radios.
Several hospitals across the Kenyan capital were Tuesday still treating the injured.
The explosion, less than half a mile from the site of the 1998 US embassy bombing, took place at 1:15 p.m. local time on Monday and appeared designed to inflict maximum damage to Nairobi’s office workers on their lunchtime break.
Kenya has seen an increase in similar attacks since October, when its military marched into neighboring Somalia in a unilateral operation designed to secure a buffer zone along the countries’ border free of Islamist terrorists.
Al Shabab immediately promised retaliations, specifically targeting “skyscrapers,” shopping centers, and major government installations.
There have been more than a dozen strikes, mostly following the modus operandi of grenades thrown into crowded areas, or improvised explosives targeting police vehicles. More than 40 people have been killed.
But “there has been nothing of the scale of Kampala,” says Andrews Atta-Asamoah, senior analyst with the Institute of Security Studies in Pretoria, South Africa. He is referring to twin bombings claimed by Al Shabab in the Ugandan capital in July 2010 that killed 76 people watching the soccer World Cup Final in two city pubs.
“What we have seen in Kenya so far appears much more likely to be individuals operating on their own and without the express orders of Al Shabab,” he says.
“The groups’ commanders may have initially wanted more spectacular attacks, but I’m getting the sense that they realize that the more sporadic and uncoordinated things are, the less chance there is for infiltration by the security services.”
Still, Monday’s explosion was significantly larger than any previous attack and appeared for the first time to be the product of a home-made bomb rather than a grenade.
Witnesses at the blast site reported a strong smell of ammonia in the minutes after the explosion.
This has led FBI agents to begin investigations into suggestions that it may have been a fertilizer bomb, which mixes fuel oil and ammonium nitrate, the AP reported, quoting official sources in Nairobi.
One stallholder, who was herself injured, told police that a “foreign man who looked Arab or Somali” had been to her shop in the minutes before the explosion and asked her to look after a bag for a short while.
A police spokesman said that investigators were “working all hours” to trace that individual.
“Terrorism does not need to be the product of major attacks, it can be a series of smaller strikes that might not leave so many people dead, but it spreads that sense of insecurity which is so corrosive,” Mr Asamoah added.
Raila Odinga, Kenya’s prime minister, struck a defiant note as he toured the blast site on Monday afternoon.
“This is a heinous act,” he told reporters. “They want to scare us. But we will not be scared.”