Nairobi, Kenya (CNN) -- It's now day four, and the nightmare still isn't over.
No one knows how many hostages might still be trapped inside Nairobi's Westgate mall, the posh shopping center that has been littered with bullets and blood since Saturday. At least 62 people have already been killed, and that number could rise.
An explosion and gunfire were heard coming from mall on Tuesday, but was not immediately clear if the blast was a controlled explosion or part of an exchange.
Several gunmen -- including snipers -- are still inside the mall, two senior officials said. And the Kenyan Red Cross said more than 60 people are unaccounted for.
But Kenya's Interior Ministry reassured a nervous public late Monday that there was little chance of escape for any surviving Al-Shabaab gunmen. It tweeted that authorities had the upper hand at the scene.
"Taken control of all the floors. We're not here to feed the attackers with pastries but to finish and punish them," Kenyan police Inspector General David Kimaiyo said on Twitter.
While the mall remains an active crime scene, authorities have also zeroed in on an airport and border crossings. More than 10 suspects were arrested at an airport for questioning in relation to the attacks,the Interior Ministry tweeted Tuesday.
"Security at all entry and exits across the country has being heightened," the ministry said.
Gunfire echoed from the mall sporadically during the Monday, sending journalists and aid workers scrambling for cover. Thick heavy smoke -- from a fire set by terrorists, according to Kenyan authorities -- billowed into the air much of the afternoon.
At least three terrorists have been killed since Saturday, the Interior Ministry said Monday. And 11 Kenyan soldiers are among the roughly 175 people wounded.
But more than 200 civilians have been rescued, the military said.
Kenyan Foreign Minister Amina Mohamed told "PBS NewsHour" that some of the attackers had come from the United States. She said they were originally from Minnesota and Missouri, PBS reported Monday.
"As you know, both the victims and the perpetrators came from Kenya, the United Kingdom and the United States," Mohamed said. "From the information that we have, two or three Americans, and I think so far I've heard of one Brit" as being among the attackers.
"The Americans, from the information we have, are young men, about between maybe 18 and 19, of Somalia origin or Arab origin," she told PBS. She offered no other specifics.
Gen. Julius Karangi, chief of Kenya Defense Forces, also said the attackers came from different countries.
"We have an idea who these people are, and they are clearly a multinational collection from all over the world," he told reporters in Nairobi. "This is not clearly a local event. We are fighting global terrorism here."
U.S. officials don't have any confirmation of Americans having been involved in the attack, Deputy National Security Adviser Ben Rhodes said.
Intelligence analysts are poring over electronic intercepts in an effort to verify the terror group's claims, two law enforcement sources told CNN.
The terrorist attack began midday Saturday in Nairobi time, with an estimated 10 to 15 gunmen taking over the mall.
Witnesses said the gunmen went from store to store, shooting people, and then took hostages.
Survivor Bendita Malakia, a North Carolina woman who moved to Nairobi in July, told CNN affiliate WAVY that she took refuge behind the closed metal gates of a store with dozens of others.
Al-Shabaab has claimed that the attackers targeted non-Muslims and vowed they would not negotiate for the hostages' lives. CNN security analyst Peter Bergen said the terrorists apparently took hostages only to prolong the siege and win more media attention.
Officials said most of the 62 dead are Kenyans. Six British citizens, two French nationals, two Indians and two Canadians, including a diplomat, also died, their governments said.
Those killed include:
-- Dutch national Elif Yavuz, a senior vaccines researcher for the Clinton Health Access Initiative based in Tanzania. Yavuz was pregnant and expecting her first child in October, according to Julio Frenk, dean of faculty at the Harvard School of Public Health. "Elif was brilliant, dedicated, and deeply admired by her colleagues, who will miss her terribly," the Clinton family said in a statement.
-- Yavuz's husband, Australian-British architect Ross Langdon. Langdon moved to Nairobi to build sustainable architecture for Africa, volunteering to build hospitals and clinic free of charge.
-- Kofi Awoonor, a renowned African poet, author and Ghanian statesman. Awoonor earned his Ph.D. from New York's Stony Brook University and was a professor of literature there in the 1970s.
-- A nephew of Kenyan President Uhuru Kenyatta, along with the nephew's fiancee.
-- A Peruvian doctor, Juan Jesus Ortiz, who had previously worked for the United Nations Fund for Children and lived in Kenya doing consulting work.
-- Sridhar Natarajan, an Indian national and employee of a local pharmaceutical firm, and 8-year-old Paramshu Jain, the son of a bank branch manager, CNN sister network CNN-IBN reported, citing officials in India.
The Somalia-based Al-Shabaab terror group said on Twitter that it had sent the gunmen to the mall in retaliation for Kenya's role in an African Union military effort against the group -- which is al Qaeda's proxy in Somalia.
Last year, the Kenyan military was part of a peacekeeping force that defeated Al-Shabaab forces to liberate the key Somali port of Kismayo.
The mall attack was the deadliest terror attack in Kenya since al Qaeda blew up the U.S. Embassy there in 1998, killing 213 people.
On Monday, Kenya's foreign minister told CNN it's clear that Al-Shabaab was not acting alone.
"This bares the hallmarks of al Qaeda," Mohamed said. "This is not just Al-Shabaab. In fact, the leaders are not Somali, as you may have heard. This was al Qaeda. It was a very well-coordinated effort."
But the attackers' national origins are irrelevant, she said.
"It doesn't matter where they come from. There are some Americans. There are some Brits. There are some others. It has nothing to do with the nationality of people. They are all evil and we must deal with them as such."