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A swarm of gunmen stormed a Kenya university before dawn Thursday, opening fire and taking hostages.

At least 15 people were killed at Garissa University College, the Kenyan Interior Ministry said. As many as 550 others are unaccounted for at the campus that had about 815 students, according to CNN affiliate Citizen TV.

The Somalia-based Al-Shabaab militant group claimed responsibility for the assault.

Gunmen burst into early morning Christian prayers, said Joel Ayora, who was on the campus and witnessed the attack. Taking hostages from the service, the gunmen then "proceeded to the hostels, shooting anybody they came across except their fellows, the Muslims."

The attackers separated students by religion, allowing Muslims to leave and keeping an unknown number of Christians hostage, Agence France-Press reported.

"We were sleeping when we heard a loud explosion that was followed by gunshots and everyone started running for safety," said student Japhet Mwala.

"There are those who were not able to leave the hostels where the gunmen headed and started firing. I am lucky to be alive because I jumped through the fence with other students."

Eventually, as many as 50 students were freed, and at least 65 people were hospitalized from the attack, the Kenyan Red Cross said.

Nine hours after the attack began, heavy gunfire and explosions continued, said Dennis Okari of CNN affiliate NTV.

Okari said he was told to take cover as hundreds of students fled, some crawling.

Security and ministry officials said one terrorist was arrested as he tried to slip through the security cordon and flee the scene.

Kenyan forces cleared three of four dormitories and had cornered the militants in the last one, the Interior Ministry said.

"This is a moment for everyone throughout the country to be vigilant as we continue to confront and defeat our enemies," Kenyan President Uhuru Kenyatta said.

He called on the inspector-general of police "to take urgent steps" to ensure that 10,000 recruits whose enrollment is pending "promptly report for training at the Kenya Police College, Kiganjo. I take full responsibility for this directive. We have suffered unnecessarily due to shortage of security personnel. Kenya badly needs additional officers, and I will not keep the nation waiting."

Waking up to terror

The gunshots started going off "like fireworks" around 5 a.m. at the time of the morning prayers, witness Milka Ndung'u told NTV. She and others escaped to a field, but gunshots followed them.

Augustine Alanga told CNN he woke up to the sound of gunfire and described students running around, seeking safety.

Assailants forced their way onto the campus by shooting at guards at the front gates, Kenya National Police said.

From there, attackers moved into a nearby girls' hostel, the Red Cross said.

It's not clear how many gunmen were on campus.

"We don't know how many there were, but there are probably more than 10," said Robert Alai Onyango, a blogger in Nairobi. "We believe the attackers were wearing something close to military fatigues."

Onyango said the attackers appeared to be shooting indiscriminately and "basically from all angles."

"They surrounded the mosque ... we don't know why they were surrounding the mosque," Onyango said.

About 300 students who escaped sought refuge at a Kenya Defense Forces camp, local newspaper journalist Steven Astariko said.

"We are saddened & angered by today's terrorist attack @ #Garissa Univ.," the U.S. Embassy in Nairobi tweeted. "Our deepest condolences 2 family/friends of victims."

Garissa is about 145 kilometers (90 miles) from the border with Somalia. Al-Shabaab militants have often launched attacks inside Kenya ever since the Kenyan government sent troops across the border to fight the group.

Credit: CNN.com