American among at least 14 killed in attack at hotel and office complex in Nairobi, Kenya

"We are under attack," one person in an office inside the complex told Reuters, before hanging up.

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By Linda Givetash and Caroline Radnofsky

An American has been identified among at least 14 people killed in a blast and heavy gunfire that sent workers fleeing for their lives at an upscale hotel and office complex in Kenya's capital on Tuesday.

The U.S. Department of State confirmed to NBC News that an American citizen was killed in the attack Tuesday.

Officials did not release his identity, but his mother, Sarah Spindler, in a phone interview with NBC News Tuesday night confirmed the American killed was her son, Jason Spindler, who she said "was trying to make positive change in the third world in emerging markets."

"We all miss him so much. And it's so sad that such a bright young person is taken away by terrorism," Sarah Spindler said.

Spindler joined the Peace Corps after the Sept. 11 terror attacks and was a graduate of NYU Law and the University of Texas at Austin, and he was the recipient of grants from the Bill and Melinda Gates Foundation and the Clinton Foundation, his mother said.

"We offer our sincerest condolences to the family and friends of this individual," a State Department official said to NBC News. "Out of respect for the family of the deceased, we have no further comment."

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The terror group al-Shabab has claimed responsibility for the attack in a statement, according to security consulting firm and NBC News partner Flashpoint Intelligence.

"We are under attack," one person in an office inside the complex told Reuters, before hanging up when the assault occurred around 3 p.m. local time.

Several vehicles were burning and gunfire continued for several minutes, witnesses said.

Security forces at the scene in Nairobi, Kenya.Thomas Mukoya / Reuters

"I just started hearing gunshots, and then started seeing people running away raising their hands up and some were entering the bank to hide for their lives," a woman working in a neighboring building said.

The Kenyan Ministry of Interior and Coordination of National Government tweeted Tuesday afternoon ET that "We have secured all the buildings affected by today's attack" and that "We are now in the final stages of mopping up the area and securing evidence and documenting the consequences of the unfortunate events." The statement was attributed to Cabinet Secretary Dr. Fred O. Matiang’i.

The ministry tweeted later that "No further threat to public exists" and "Civilians who had been secured in one building by security while mopping up continues now safely evacuated." The tweet said "All buildings and surrounding area secure."

"We condemn this senseless act of violence," a U.S. State Department spokesman said. "The U.S. Embassy has actively offered assistance to local authorities."

Nairobi suffers from violent robberies but has also been targeted by Somali Islamist militants.

Nairobi's Westgate Mall was attacked in 2013, when al-Shabab extremists burst into the luxury shopping center, hurling grenades and starting a days-long siege that left 67 people dead. The mall is less than two miles away from the site of Tuesday's incident.

The attack came three years to the day after al-Shabab extremists attacked a Kenyan military base in neighboring Somalia, killing scores of people.

An expert on the terror group told NBC News that it had appeared Kenyans were managing the threat of terrorism well in the years since the last attack, but Tuesday's incident proves there was "a blank spot."

"I think it's a very clever target," said Stig Jarle Hansen, author of "Al-Shabaab in Somalia." "Lots of foreigners live in the area. It's not an area with high security though."

Targeting a hotel could have longer-term implications for the country, Hansen added. "This is hitting the main artery of Kenyan export earnings: the tourist industry."

Reuters, Associated Press, Janelle Griffith, Doha Madani and Brooke Glatz contributed.