The accident at Japan’s Fukushima Daiichi plant in 2011 alerted the American nuclear industry and its regulators to the possibility that operators at plants with more than one reactor might have to deal with more than one meltdown at a time in a flood, earthquake or other catastrophe. Officials are now working to assure that they could master that situation.

But documents uncovered by a group that is critical of nuclear safety show that a high-level safety analyst at the Nuclear Regulatory Commission posed the possibility to his superiors in July 2007, about four years before the earthquake and tsunami that led to three simultaneous meltdowns at Fukushima Daiichi. The documents also show that in August 2008, the commission staff formally acknowledged the issue.

But until Japan’s disaster, progress in the American nuclear industry was glacial. Dave Lochbaum, a nuclear expert at the Union of Concerned Scientists, which uncovered the documents, compares them to records located after the crash of the space shuttle Columbia in which engineers voiced concern that debris falling during a launch could damage an orbiter.

Action to prepare for a dual meltdown was not a case of “forewarned is forearmed,” he said, but more like “forewarned is forestalled.”

The warning, which now seems prophetic, predicted “common cause failures,” meaning single events that disable different pieces of equipment that are supposedly independent and nearly invulnerable to failing simultaneously on their own. The risk analyst, Richard Sherry, wrote that flooding or earthquakes could disrupt both normal grid power and emergency backup power.

Mr. Sherry was concerned with a “station blackout,” or S.B.O., which is what eventually happened at Fukushima. He wrote that a station blackout at more than one unit “challenges the ability of the plant operating staff to respond and may require resources (technical staff and equipment) that exceed what is currently available (e.g. diesel driven pumps, portable DC generators) based on the assumption that only a single unit experiences an S.B.O.”

There are 36 sites in the United States with more than one reactor operating. Some have three, and Plant Vogtle near Augusta, Ga., has two operating with two more under construction.

Of course, this is not quite an “I told you so” moment, given that the multiple meltdowns were in Japan and not at any site under the jurisdiction of the Nuclear Regulatory Commission. But Mr. Lochbaum said that the memo suggests that the agency is better at identifying safety problems than resolving them.

Before Fukushima, it moved to put the multiple meltdown issue into a category called “generic safety issues,” which are questions that do not fit neatly into the agency’s regulatory framework. Mr. Lochbaum and his organization have complained in the past that one such “issue,” concerning whether debris generated in an accident would clog emergency pumps,has been unresolved for more than 15 years.

A commission spokesman, Scott Burnell, cast the 2007 memo differently. Mr. Sherry was not the first to identify “the very small possibility of simultaneous reactor severe accidents at multi-unit sites,” he said in an e-mail, and the agency has been pursuing the issue, “incorporating new information as time went on.”

Mr. Lochbaum disagreed. “Had the N.R.C. acted in a timely fashion in August 2007 when Sherry nailed the issue on its head, implementation would be done by now,” he said. Saying that was impressed by Mr. Sherry’s forethought, he joked that he wanted to call him to see if he could predict winning lottery numbers.