"Let's be clear, as New Yorkers, our lives revolve around the subways," Mayor Bill de Blasio said on Monday morning after a would-be terrorist set off a ``low-tech’’ device in a subway passageway in Midtown Manhattan.

"When we hear of an attack on the subway, it's incredibly unsettling and let's be also clear this was an attempted terrorist attack," he said at a news conference.

The suspect, Akayed Ullah, was one of four people injured in the explosion, which occurred at about 7:20 am in a subway passageway near 42nd Street and 8th Avenue, in Midtown Manhattan near Times Square, police said.

There have long been concerns that the subway system could be a terrorist target. Passengers at some stations are subject to random searches of their bags by police, and the subways have had an increase in police presence. But the system is a particularly soft target because it has hundreds of stations, and people are closely packed together during morning and evening rush hours.

Close to 6 million people rode the subways on an average weekday in 2016, according the Metropolitan Transit Authority (MTA), which oversees the system’s operation. Total ridership in 2016 was 1.757 billion.

In October, federal authorities said three men living outside the US planned to attack concerts, Times Square and crowded subways in 2016 in an ISIS-inspired attack that one of the suspects said he hoped would be "the next 9/11’’. The plot was foiled with the help of an undercover FBI agent, officials said.

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