Security at major airports and train stations across the US has been further stepped up in the wake of the tragedy in Brussels and as concerns spread over the possibility of a copycat attack.
Shortly after the attacks, Homeland Security Secretary Jeh Johnson said the US administration was bolstering the police presence at potential targets, in particular airports and railway stations.
"TSA (Transportation Security Administration) is deploying additional security to major city airports in the United States, and at various rail and transit stations around the country," he said.
"TSA is also working closely with state and local law enforcement, airport and transit authorities, and the aviation industry in order to augment that security."
Indeed, reports from around the country show a stepped-up security presence at major airports.
In New York, for instance, there were large numbers of uniformed police officers and National Guard troops in battle armour patrolling the terminals.
At various subway stops, police conducted random bag checks. There were also messages being played constantly over the public address system urging commuters to report suspicious packages.
In Washington, passengers reported getting stopped by officers conducting random checks.
Consultant Nisha Rai, 35, who arrived in Washington from Amsterdam, said: "I put my passport away after immigration but just trying to go to bathroom and get out of the airport, police kept coming up, asking to see my passport, and asking me what I was doing in the country and how long I would be staying."
For now, security experts say there are no credible threats against US targets, and few are calling for screening everyone entering an airport.
This article was first published on March 27, 2016.
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