Daniel Kammen, an energy scientist specializing in renewable power sources, has resigned as one of the State Department’s science envoys over President Donald Trump’s “failure to condemn white supremacists and neo-Nazis.” In his resignation letter, which he posted to Twitter Wednesday morning, he appeared to leave a coded message.

Kammen was, until Wednesday, one of seven scientists the State Department charged with building diplomatic research and development connections abroad. He’s also a professor of nuclear engineering at the University of California, Berkley. As reporter Yashar Ali spotted, the first letter of each paragraph of his letter spelled out a coded word: IMPEACH.

Science envoy is not a particularly powerful position in the U.S. government, but it’s considered a real honor to take it. Envoys are often researchers of enormous prestige, who’ve done well-known research or received acclaim for their work.

You can read Kammen’s full letter in his tweet below.