As New Yorkers made their way to work, there was a tangible air of sobriety on the city's commuter trains during the morning rush hour.
Front-page photographs in every newspaper of the carnage in Madrid were hard to avoid, giving people here a chilling echo of 11 September and raising fears about another terror attack.
Spaniards and New Yorkers came together to mourn
One newspaper, the New York Daily News, printed the words "Spain's Sept 11" on its front page.
The bombings in Madrid have prompted new precautions here in New York.
The mayor, Michael Bloomberg, said that security had been beefed up at train and subway stations across the city.
It is clear that the authorities here are taking no chances.
"We cannot afford to let our guard down", Mr Bloomberg said.
"It goes to show that we still live in a very dangerous world.
"And we are clearly focusing, as you would expect, even more resources on the New York City subway system," he added.
Armed police officers and members of the National Guard at busy transport hubs such Grand Central Station are a feature of life for New Yorkers.
For many here they are a reassuring presence.
Ever since the 11 September attacks, the city has remained on a heightened state of alert.
In Washington DC, subway platforms were cleared of rubbish bins and containers as officials tried to remove any potential hiding place for a possible bomb.
New Yorkers know all too well the suffering Madrid has endured
Amtrak, the national train service, increased patrols of its police force.
Although extra, visible measures are now in place, US Department of Homeland Security officials have not raised the colour-coded threat level, which currently stands at yellow.
'Pain and outrage'
On Thursday, when Americans woke up to the news of the attack in Madrid, coverage was far from comprehensive on domestic television channels.
But as the scale of the atrocity became clear - coupled with a possible al-Qaeda link - US newspapers voiced strong condemnation and emotional solidarity with the victims in Spain.
The Washington Post called Spain "a true and valuable ally" in the war on terrorism and said Madrid had "suffered a blow as shocking and terrible as any the enemy has landed since September 11, 2001".
For the New York Post, New York and Madrid are now sister cities "united in pain and outrage, and gripped with a determination now to not let the terrorists win. Not ever".
The New York Times ended its editorial with the words: "We are all Madrilenos now."