The study concluded that failure to bolster defenses against hackers could lead to disruption of the computer networks used to move goods, fuel and food from ships to the marketplace.

"Shelves at grocery stores and gas tanks at service stations would run empty," the study said. A halt in "energy supplies would likely send not just a ripple but a shock wave through the U.S. and even global economy."

In addition to Baltimore, Kramek examined California's ports in Los Angeles and Long Beach, Houston and Beaumont in Texas, and Vicksburg on the Mississippi River.

But Baltimore port officials and their West Coast counterparts in Long Beach disputed some of Kramek's findings, which were released last week by Brookings.

Port of Baltimore spokesman Richard Scher called the 50-page study "misleading and factually incorrect."

Kramek concluded that "the cybersecurity culture is not high" at the Maryland Port Administration, which oversees the port of Baltimore. He said port officials have not conducted a threat assessment or developed a response plan. Further, he said, port officials had not applied for federal grants to carry out a cybersecurity project.

Kramek wrote that a successful attack on the computer systems of Baltimore's port or its tenants "would quickly disrupt cargo operations and slowly ripple out to impact the one-third of the U.S. population that resides within an overnight drive" of the port.

The port, ranked 12th in cargo tonnage in the nation, has been unsuccessfully attacked by hackers, but because the facility's computer system is part of the Maryland Department of Transportation network, the target was unclear, the study said. Attempts to break into the port's wireless network were blamed on crew members aboard visiting ships trying to access free WiFi.

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