Troops are out, patrolling the city of Boston, and the mood evokes the early hours after the 2001 terrorist attacks. This time, though, Americans are less surprised.
Deborah Kathan, a 57-year-old orthopaedic surgeon, heard what she calls a "pop, pop," while standing near the finish line of the Boston Marathon.
"I thought, 'It's like New York all over again.'"
She was working in a Minneapolis hospital when the 2001 attacks occurred. She watched images of the burning buildings on TV.
"It's not that I was afraid. It was almost like you were expecting it," she says. "And it did happen again - only it wasn't so bad this time."
At least three people were killed, and scores injured in the Boston explosions. (Nearly 3,000 people were killed on 9/11). Officials have not yet explained the cause of the blasts.
"We still do not know who did this or why. And people shouldn't jump to conclusions before we have all the facts," President Barack Obama said after the explosions. "But make no mistake - we will get to the bottom of this."
Central Boston remains a place of chaos and confusion - much like New York and Washington in 2001.
Around Copley Square, where the blasts hit, uniformed men walk the streets looking through buildings. Some, like the pair spotted on an escalator at the Westin hotel, carry automatic rifles.
Some have handguns on their holsters - or are wearing bulletproof vests. They are looking for the perpetrators and are trying to make sure there are no more explosions.
"Busy day," says a Massachusetts National Guardsman as he walks into the Westin.
He and other Guardsmen are here alongside the FBI, the Massachusetts state police, the city of Boston police, officials from the bureau of Alcohol, Tobacco and Firearms, local transit police - and others who are less easily identified.
"We believed we had turned the corner with these kinds of threats," says Georgetown University's Bruce Hoffman, author of Inside Terrorism. "But once again we see terrorist violence can occur in a major city.
"If you want to kill to get publicity, go to the Boston Marathon," he says. "The shock, the horror - it's a message that will have greater impact."
In Bar 10, a place at the Westin hotel where Deborah Kathan is hanging out with friends, TV screens flash a phone number, the mayor's hotline, where people can call to get information about the attacks.
The place has a feeling of uncertainty - two people at the bar, for example, are wondering whether they will be able to get back into their own hotel, closed because of a bomb scare.
Alcohol helps. Someone hands Ms Kathan a glass of red wine, and she recalls some of the details of the afternoon. "I saw the smoke, and they kept telling us, 'Move out, move out.'"
"There was like a plasticky smell - burning. I kept saying to these guys -" she looked over at her boyfriend, Ken Ekman, who had run the marathon, and their friend Chris Bray, who had also completed the race before the explosions.
"I said, 'Don't you smell that smell?' And they were like, 'No'" she recalls.
"No, we didn't," Mr Bray. "'Cuz we stunk." He was joking - sort of. He had finished the marathon in 2 hours 54 minutes and 58 seconds, he says - and had not yet showered.
As on 9/11, warnings were announced - then turned out to be false alarms.
Another runner, Dave Jacobs Robinson, says he got close to the finish line when he was told to stop. He wandered around and tried to go back to the Westin.
He had left the hotel at 06.20 - but did not get back till after 19.00 (EST). "The place was cordoned off," he says.
Employees at the hotel - like at other places in the area - had been told a bomb might go off on the premises. For a period of time, they would not let anybody in.
Still, the afternoon was not all bleak. One of Ms Kathan's friends, a landscaper named Rob Tuprin, says he is "still kind of in shock". He says he ran the marathon in three hours eight minutes, though.
"Not my fastest," he says, holding a glass of white ale. "But also not my worst."
As he talks, a blonde woman approaches him and asks how he feels after the race. They chat for a while - then she wanders off. He glances in her direction. "Terrible things do bring people together," he says.
Deborah Kathan says that she still feels the shock, at least a bit. Still she knows that it may not be the last time.
"It will happen again," she says. "But we're tough."