The U.S. Embassy in Tokyo on Monday notified U.S. citizens living in Japan that it had received a threat over the Internet about a possible bomb attack this week.

Embassy officials said Monday afternoon that they had yet to determine whether the threat was credible.

“It was received through the Internet. I don’t have the specifics,” said an embassy spokeswoman who asked not to be identified by name.

As a matter of policy, embassy officials said, they would not comment on any extra security measures. But they did note that it was business as usual at the complex Monday, with employees reporting to work as scheduled.

The embassy e-mailed the notice Monday through its warden system, a list of registered American residents of Japan and organizations known to have U.S. employees or students, said the spokeswoman.

The notice read in part: “The U.S. Mission in Tokyo has received information, through the Internet, about a possible bomb attack against the American Embassy during the week of May 10. We are unable at this time to determine the credibility of the threat but we feel it is our responsibility to take all such information seriously and to share this with the American community.”

The embassy spokeswoman stressed the e-mail was a notice, not a warning. “A warning is a specific threat. We don’t know the credibility of this. It is sharing information that we have received.”

The e-mail notice also said that “the appropriate Japanese authorities have been notified.”

But spokesmen for the Metropolitan Police Department in Tokyo and the police station in Akasaka, the Tokyo neighborhood housing the embassy, said Monday they were unaware of the bomb threat.

“We have no knowledge of such an incident here” at the public relations office level, said a Metropolitan Police Department spokesman.

“However, we are not saying whether or not an investigation is under way,” he added, declining to comment further.

U.S. Forces Japan spokeswoman Capt. Richelle Dowdell said Monday, “We have not received any threats that we are aware of at this time” against U.S. military installations in Japan.

Monday’s Internet threat against the U.S. Embassy in Tokyo was the second since late April, though the first appears to be a hoax, Chiba prefectural police have indicated. They are investigating a 21-year-old college student for posting the April 27 Internet message on “2 Channel,” a popular Japanese Internet bulletin board. A police spokesman at Funabashi Higashi Police Station said the student turned himself into police about 30 minutes after writing the message. “He told police that he became scared after jokingly posting the message from his home computer at about 12:10 p.m. on the day,” the official said.

“He told investigators that he wrote the message after he saw a similar threat against the Chinese Embassy,” the police spokesman said. “But after posting his message, he hurried to the police, realizing that he could be in a big trouble.

“He told police that it was a joke and the threat was groundless.”