The US embassy in Tokyo has received an e-mail threatening a bomb attack this week, and officials have warned American citizens in Japan, the embassy said today. The notice said the threat had arrived "through the internet," but an embassy official said the bomb threat was sent to authorities via e-mail. The threat said the attack would come the week of May 10. Embassy officials refused to say if any additional security steps had been taken, though Japanese police had been fully informed. The notice to Americans did not spell out any steps they should take.