Maryland had not conducted a recommended risk assessment on the Baltimore bridge that collapsed after it was struck by a ship last March, the National Transportation Safety Board said on Thursday.

In a news briefing, the board’s chairwoman, Jennifer L. Homendy, said if Maryland officials had conducted the assessment, they would have found that the bridge, the Francis Scott Key, was at serious risk of collapse from a strike by a large ship.

Six workers on a crew doing overnight repairs to the bridge roadway died in the collapse, which paralyzed one of the nation’s busiest ports for days.

Dozens of other bridges across the country, including landmarks like the Brooklyn Bridge, have not had the same recommended assessment, and thus carry an unknown level of risk, Ms. Homendy said.

“Frankly, we’ve been sounding the alarm on this since the tragedy occurred,” she said at the news conference in Washington.

Investigators with the board found that dozens of bridges in 19 states had not been assessed for risk of collapse in recent years, even though the volume of vessel traffic passing underneath these bridges suggested a strike by a ship was a distinct possibility. The N.T.S.B. has contacted the operators of these bridges — in most cases, state transportation departments and regional port authorities — to urge them to conduct assessments and, if needed, come up with measures to reduce the risk of collapse.

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