WASHINGTON (REUTERS) - Atlanta's airport was briefly evacuated on Wednesday over a suspicious package while US law enforcement agencies and travellers were on edge a day after deadly suicide bombings by Islamist militants rocked Brussels.
Passengers were ordered out of public areas of the domestic terminal at Hartsfield-Jackson International Airport, the United States' busiest by passenger volume, but the site was quickly cleared and operations resumed, airport officials said.
Parts of Denver airport were also evacuated on Tuesday, hours after at least 31 people were killed and 260 wounded in attacks on Brussels airport and a rush-hour metro train, as airports across the United States tightened security.
US officials were trying to find Americans missing after the attacks, which the officials said injured about a dozen US citizens including three Mormon missionaries, a US Air Force airman, and four members of his family.
"We are still assessing the impact," US State Department spokesman Mark Toner said in a statement. He could not confirm whether any Americans were killed.
"We must emphasise that a number of US citizens remain unaccounted for," Toner said.
Representative Devin Nunes of California, chairman of the US House intelligence committee, said the attacks may have been aimed at US citizens, noting that the airport blast struck close to US airline counters and that the metro station hit was near the US embassy.
"It looks like it was targeted toward Americans to some degree," Nunes told reporters.
Apart from the eight Americans confirmed as wounded, US media reported on Wednesday that relatives of at least four other Americans who had been travelling in Belgium were still trying to track them down.
Husband and wife Justin and Stephanie Shults, originally from Tennessee and Kentucky, respectively, but now living in Belgium, have not been heard from since they dropped a relative at the airport shortly before the blasts, a family member said.
"We haven't been able to contact them going on 30 hours," Justin Shults' brother, Levi Sutton, told Reuters in a Facebook message. "Stephanie's mom is fine but she was separated from Justin and Stephanie."
DEATH TOLL COULD RISE
Sister and brother Sascha and Alexander Pinczowski, who had been living in New York, remain unaccounted for, the New York Daily News reported. The Pinczowskis' citizenship was unclear.
A woman who identified herself on social media as Alexander Pinczowski's girlfriend said she had been unable to contact him since Tuesday morning.
Belgian officials have said the toll from the bombings could increase because some victims at the subway station were blown to pieces and hard to identify, and several survivors were still in critical condition.
The Church of Jesus Christ of Latter-day Saints said on Wednesday that one of its missionaries, Richard Norby, 66, was in a medically-induced coma after undergoing lengthy surgery to address shrapnel wounds and second-degree burns.
The attacks sent shockwaves across Europe and around the world, with authorities racing to review security at airports and on public transport systems.
Islamic State in Iraq and Syria (ISIS), which controls areas of Syria and Iraq and has sympathisers worldwide, claimed responsibility for the Brussels bombings, fuelling debate and controversy in the United States about how to stop such attacks.
US Republican presidential campaign front-runner Donald Trump has advocated torturing suspects to obtain information on militant plans.
Another Republican candidate, US Senator Ted Cruz of Texas, called for heightened police scrutiny of neighbourhoods with large Muslim populations.
President Barack Obama, a Democrat, rejected singling out Muslims in the fight against ISIS and said while on a visit to Argentina that any such approach "is not only wrong and un-American, but it also would be counterproductive because it would reduce the strength, the antibodies that we have to resist the terrorism."
Obama and Vice-President Joe Biden said the United States was offering Belgium all assistance to help bring the bombers to justice.
US Secretary of State John Kerry will visit Belgium on Friday, a State Department spokesman said.
While major transportation hubs in the United States were on high alert, Homeland Security Secretary Jeh Johnson has said there were no known specific and credible threats to the country.