Story highlights (updated 9/22 17:09 EDT):

Kenyan military announces "most of the hostages have been rescued" after operation to end siege

Hostages inside a Kenyan mall were being held for the second day after gunmen opened fire on Saturday, Sept. 21

At least 68 people are reported dead and about 30 hostages were inside

The Somalia-based al-Shabab group claimed responsibility for the attacks

At least three of the attackers are from the United States, according to CNN

The Kenyan military has announced that "most of the hostages have been rescued" and security forces have taken control of most of the Westgate shopping mall, the site of a deadly terrorist attack that began Saturday and turned into a two-day hostage crisis.

“Most of the hostages have been released, and the Kenya Defence Forces has taken control of most parts of the building,” Kenyan military spokesman Colonel Cyrus Oguna told the television station KTN, according to Reuters. He did not elaborate on how many hostages had been held or freed.

Security officials said a major operation had begun late Sunday and the standoff would end "tonight," according to AP. The Kenyan military later reported on Twitter that four soldiers had sustained injuries in the operation that freed many of the hostages.

As many as 30 hostages were reportedly being held inside and at least 68 people were killed in the attack, Kenyan Red Cross said in a tweet.

Attackers, armed with Ak-47 rifles, machine guns and grenades, had been holding off police from inside the mall's Nakumatt supermarket, one of Kenya's biggest chains, according to the Chicago Tribune.

Joseph Ole Lenku, a senior Kenyan Interior Ministry official, said that roughly 10 to 15 militants were left inside.

The Somalia-based al-Shabab group claimed responsibility for the attack, which they said was in response to the presence of Kenyan troops operating inside Somalia.

Sources told CNN that two of the attackers are originally from Minnesota and one is from Missouri. Al-Shabab has "an active recruitment and radicalization network inside the U.S." and had at least 40 or more American citizens fighting for them in 2011, according to a Committee on Homeland Security investigation.

USA Today reported a large explosion inside the mall on Sunday, but the source of the blast was unclear.

"They Kenyan govt is pleading with our Mujahideen inside the mall for negotiations. There will be no negotiations whatsoever," the group posted on to Twitter on Saturday. The account was suspended Sunday morning.

The attack "appeared designed to achieve maximum global impact by targeting a place frequented by Westerners as well as Kenyans," according to Reuters.

Officials have confirmed that three British nationals were killed in the attacks. Two French citizens and two Canadians were killed, the governments said, according to Reuters.

The U.S. State Department said four Americans were believed to be injured in the attack.

Below are photos taken at the Westgate shopping center where the attacks took place: