The same extremist group that took over a Nairobi mall attacked a security post near the Somali border on Thursday and killed two police officers, while promising the violence would continue until Kenyan troops were withdrawn from Somalia.
The leader of the Somalia-based al-Shabaab, Ahmed Godane Shaykh Mukhtar Abu Zubayr, warned the Kenyan public there was no way they could "withstand a war of attrition inside your own country," in a new statement posted on the Internet late on Wednesday.
"Make your choice today and withdraw all your forces,..." Godane said. "Otherwise be prepared for an abundance of blood that will be spilt in your country, economic downfall and displacement."
The early morning attack on the town of Mandera was the second by al-Shabaab terrorists following the bloody four-day standoff at the Westgate Mall that ended on Tuesday in which at least 67 people were killed.
Regional police chief Charlton Mureithi said in addition to the two police officers killed, three others were injured and 11 vehicles destroyed in Mandera.
On Wednesday night, terrorists attacked the border town of Wajir. One person was killed and four wounded after a gunman opened fire and threw grenades.
Such attacks occur regularly but have taken on new significance following the Westgate Mall attack.
In his statement, Godane said the mall attack was not only directed at Kenya, but was also "a retribution against the Western states that supported the Kenyan invasion and are spilling the blood of innocent Muslims in order to pave the way for their mineral companies."
Forensic experts from around the world, including the US, Britain and Germany, continued their work on Thursday reconstructing what is now considered a crime scene, carrying out fingerprint, DNA and ballistic analysis, said police spokeswoman Gatiria Mboroki.
She said she had no details on what the experts had found so far in the bullet-scarred, scorched mall and that their work was expected to take a week.
Washington is providing technical support and equipment to Kenyan security forces and medical responders, said US Ambassador Robert Godec. The US is assisting the investigation to bring the attack's organizers and perpetrators to justice, he said on Wednesday.
At least 18 foreigners were among those killed when the terrorists entered the Westgate Mall on Saturday, firing assault rifles and throwing grenades, including six Britons and citizens from France, Canada, the Netherlands, Australia, Peru, India, Ghana, South Africa and China.
In addition to those killed at the mall, another 175 people were injured, including more than 60 who remain hospitalized.
Morgue officials in Nairobi have been preparing for a large influx of bodies still in the mall. Officials have told The Associated Press that the shopping center could hold dozens more bodies.
Authorities have said at least five al-Shabaab attackers were killed and another 11 suspects have been taken into custody.
In his statement, Godane said only that "some" of his fighters had been killed, possibly suggesting that others escaped.
A gaping hole in the mall's roof was caused by Kenyan soldiers who fired rocket-propelled grenades inside, knocking out a support column, a government official told The Associated Press. The official, who insisted he not be identified because he was sharing security information, said the soldiers fired to distract a terrorist sniper so hostages could be evacuated.
Video of the roof collapse showed massive carnage. The collapse came on Monday, shortly after four large explosions rang out followed by billows of black smoke. Although a government minister said the terrorists had set mattresses on fire, causing the roof to collapse, the video showed such massive destruction that the explanation seemed unlikely to be the full story.
Al-Shabaab, whose name means "The Youth" in Arabic, first began threatening Kenya with a major terror attack in late 2011, after Kenya sent troops into Somalia following a spate of kidnappings of Westerners inside Kenya.
The mall attack was the deadliest terrorist attack in Kenya since the 1998 al-Qaida truck bombing of the US Embassy in Nairobi, which killed more than 200 people.