NEW YORK (AP) — With a blast that made skyscrapers tremble, an 83-year-old steam pipe sent a powerful message that the miles of tubes, wires and iron beneath New York and other U.S. cities are getting older and could become dangerously unstable.

The steam conduit that exploded beneath a Manhattan street at the height of rush hour July 18, just a block from Grand Central Terminal, was laid when Calvin Coolidge was president, and was part of a system that began providing energy to city buildings in 1882.

The explosion killed one person and injured dozens, some seriously.

Investigators are still trying to determine the explosion’s cause, but some experts said the age of the city’s infrastructure was a possible factor.

“This may be a warning sign for this very old network of pipes that we have,” said Anil Agrawal, a professor of civil engineering at the City College of New York. “We should not be looking at this incident as an isolated one.”

From Boston to Los Angeles, a number of American cities are entering a middle age of sorts, and the infrastructure propping them up is showing signs of strain.

The American Society of Civil Engineers estimates it will take $1.6 trillion over the next five years to get the nation’s roads, bridges, dams, water systems and airports into good condition.

But replacing old parts in a labyrinth of cables, tunnels and piping, often extending hundreds of feet down, is rarely easy.

“The fact that all of this stuff is crowded together in a very small space can also make accidents worse,” said Rae Zimmerman, director of the Institute for Civil Infrastructure Systems at New York University.

“When one thing goes, other things go. When you have a water main break, it will wash out a street and break a gas line,” she said.

In New York City, home to the largest steam system in the world, steam is pumped through more than 100 miles of mains and service pipes to customers such as the Empire State Building and Rockefeller Center. It is also widely used by dry cleaners and hospitals.

Consolidated Edison, the utility that operates the steam system, insisted its equipment is in good shape. The company said it is spending $20 million this year on upgrades, and has been removing older cast-iron components, eliminating asbestos from manholes and installing improved joints less likely to fail.

A dozen air samples showed the explosion did not leave asbestos in the air, Mayor Michael Bloomberg said.

Still, officials were asking residents to be cautious and to turn in their dust-covered clothes to emergency crews. A yellow tape blocked off a zone of several square blocks surrounding the site.

Some speculate that rainwater or water from a main break somehow seeped onto the pipe, and the sudden interaction between cold water and super-hot steam burst the conduit.

Con Ed said some components of the system are examined about every six weeks, but steam mains underground are generally not inspected because doing so often requires digging up the street.

That is something that should change immediately, Agrawal said. Robotic probes can detect corrosion or damage to steam pipes from within, without having to dig them up, he said.