"Kenyan cities will run red with blood," said al-Shabab according to the SITE intelligence monitoring group.
The Islamic militants said the attack on Garissa college was in retaliation for killings carried out by Kenyan troops fighting the rebels in Somalia.
"No amount of precaution or safety measures will be able to guarantee your safety, thwart another attack or prevent another bloodbath," said al-Shabab.
Following the extremists' threats, Kenyan President Uhuru Kenyatta vowed to take harsh measures against the Islamic militants.
"We will fight terrorism to the end," said Kenyatta. "I want you to know that our security forces are pursuing the remaining accomplices. We will bring all of them to justice," he said.
"We are also in active pursuit of the mastermind (of the Garissa attack) and have placed a reward for his capture." Kenyatta declared 3 days of national mourning over the Garissa attack.
Kenyan security agencies arrested three people trying to cross into Somalia, said Interior Ministry spokesman Mwenda Njoka in a Twitter post. He said the three are associates of Mohamed Mohamud, also known as Dulyadin Gamadhere, a former teacher at a Kenyan Madrassa Islamic school who authorities say coordinated the Garissa attack.
Kenyan authorities have put a USD 220,000 bounty for information leading to Gamadhere's arrest.
Two other suspects were arrested at Garissa college. A survivor of the killings at Garissa University College was found today, two days after the attack by Islamic extremists killed 148 people.
She was rescued shortly before 10 am, according to Kenyan officials.
