WASHINGTON (Reuters) - Atlanta’s airport was briefly evacuated on Wednesday over a suspicious package while U.S. law enforcement agencies and travelers were on edge a day after deadly suicide bombings by Islamist militants rocked Brussels.

The lights of the Empire State Building are darkened in honor of the victims of the Belgium attacks in Brussels, at the Manhattan borough in New York, March 22, 2016. REUTERS/Stephanie Keith

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 271 wounded in attacks on Brussels airport and a rush-hour metro train, as airports across the United States tightened security.

U.S. officials were trying to find Americans missing after the attacks, which the officials said injured about a dozen U.S. citizens including three Mormon missionaries, a U.S. Air Force airman, and four members of his family.

Among those missing were U.S. government personnel, a State Department spokesman told reporters in Washington.

“We still have not accounted for every official U.S. government employee or their family members on the ground,” said the spokesman, Mark Toner. “Partly that reflects the size of the mission or three missions: there’s a bilateral mission, there’s a mission to the EU, as well as a mission to NATO.”

The situation, Toner added, remains “very fluid.” He could not confirm whether any Americans were killed.

Representative Devin Nunes of California, chairman of the U.S. House intelligence committee, said the attacks may have been aimed at U.S. citizens, noting that the airport blast struck close to U.S. airline counters and that the metro station hit was near the U.S. embassy.

“It looks like it was targeted toward Americans to some degree,” Nunes told reporters.

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Apart from the eight Americans confirmed as wounded, U.S. media reported on Wednesday that relatives of at least four other Americans who had been traveling 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 death toll could increase because some victims at the subway station were blown to pieces and hard to identify, and several survivors were 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 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, which controls areas of Syria and Iraq and has sympathizers worldwide, claimed responsibility for the Brussels bombings, fueling debate and controversy in the United States about how to stop such attacks.

U.S. Democratic presidential front-runner Hillary Clinton said the United States and Europe should take a “harder look” at protocols at airports and other “soft sites” outside security perimeters.

U.S. Republican presidential campaign hopeful Donald Trump has advocated torturing militant suspects to obtain information, while another Republican candidate, U.S. Senator Ted Cruz of Texas, called for heightened police scrutiny of neighborhoods with large Muslim populations.

President Barack Obama, a Democrat, rejected singling out Muslims 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. U.S. Secretary of State John Kerry will visit Belgium on Friday, a State Department spokesman said.