The man, Sharif Mobley, who is charged by Yemeni authorities with murdering a hospital guard during an escape attempt, said he told others in his union: “We are brothers in the union, but if a holy war comes, look out,” said the report from the inspector general, Hubert T. Bell.

Fellow nuclear plant workers said Mobley had referred to non-Muslims as “infidels” and had visited “unusual” Web sites on his personal computer, including one showing a mushroom cloud the report said.

The American-born Mobley, 26, worked between 2002 and 2008 as a laborer at six nuclear plants in New Jersey, Pennsylvania and Maryland.

Mobley told friends in 2008 that he planned to move to Yemen to study Arabic and Islam. American and Yemeni officials said he connected there with Al Qaeda in the Arabian Peninsula, the branch of the terrorist network in Yemen.

Mobley was arrested by Yemeni security officers in January and was taken to a hospital in Sana, the capital, after complaining of illness. On March 7 he grabbed a security guard’s gun and tried to escape, fatally shooting one guard. He remains in Yemeni custody on capital murder charges.

His lawyers have admitted that Mr. Mobley was in contact with Anwar al-Awlaki, an American-born radical cleric in Yemen who has been linked to several terrorist plots and has called for attacks on the United States. But the lawyers have said Mr. Mobley denies being involved in any terrorist plot.