The group claims American navy was trying to get details of a report on how dolphins trained for military use in the former Soviet Union have been sold off since the end of the Cold War.
The old Soviet navy used highly-trained dolphins because their natural sensory systems made them ideal for military use.
But at the end of the Cold War they became expensive to keep and conservationists claim they were sold to willing buyers around the world, at least from the Sevastopol naval base in the Ukraine.
The information in the report by the Whale and Dolphin Conservation Society, the WDCS, has been kept secret on a computer file pending the release of the report.
But a week ago the group noticed that someone somewhere was trying to hack into the file. The trail was easy to follow. The crude attempt to get information was made by the US military.
Computer experts say it was not simply a slip of the finger.
"The kind of information they have received gives more information about the internal workings of our computer systems.
"Effectively that will allow you to get into the further areas of the site, if you wanted to," says Matt Penton from Merchant Technologies.
The Americans have also used dolphins for military work. But according to the report, there was some concern that a gap was emerging. The Soviets apparently had better trained dolphins than the Americans.
The WDCS report details how the Soviet military trained dolphins were sold to several countries, including Argentina, Hungary and Yugoslavia.
The group says the Americans twice asked for a copy before publication and they criticise the method of trying to get the information.
"We are actually very concerned that someone like the US navy feels like they have come through the back door to try and break in here," says Chris Stroud from the WDCS.
"We are not going to try and pursue them through the courts but we are looking for some explanation. We have actually asked the US embassy for a rational explanation of this activity," he says.
"We will give them the report, but they just have to wait like everybody else."
The Americans on Tuesday admitted a call had been made from a computer in the Pentagon. But they have not yet established who was responsible and what their motives were.
But the report from the dolphin charity will suggest that some of these highly intelligent creatures ended up at dolphinariums to entertain the public.
Others, though, may have continued their military careers elsewhere and that would be useful information for the intelligence services.