This is a common claim of people selling "free energy" devices that the government or oil companies are trying to suppress their work.
I'm wondering if there's any documented examples of an oil company or government going after someone, suing them, killing them, whatever - because they invented a competing energy technology that's too cheap to sell or tax. I am not able to find such an incident.
I don't believe this claim. I think it would basically be impossible to not make money from such a technology, and the market would overcome any possible attempts to keep such a thing under wraps.
Here's some sketchy sites making the claim:
Free Magnetic Energy cites a law suggesting the US Government has a legal right to keep inventions secret.
Water Powered Car [WARNING: Site plays music.] suggests that previously inventors were mysteriously killed to suppress their work, and that the government would lose its gasoline taxes.