Five aluminum bottles strapped with fireworks and apparently filled with kerosene were thrown into the U.S. Embassy compound in Minato Ward, Tokyo, early Saturday, but they did not explode, police said.
They said Monday they were examining the bottles to see whether they were designed to explode. They said groups opposed to last year's U.S.-led invasion of Iraq might have been responsible.
They said the incident is being treated as a possible violation of the Minor Offense Law.
Saturday marked the anniversary of the invasion that divided the international community, with tens of thousands of people nationwide staging protests.
Police said the bottles were thrown into the grounds to the right of the main gate shortly before 4 a.m., apparently from a building across the street.
Riot police guarding the entrance of the embassy heard a strange noise and notified embassy security personnel, who found the bottles, police said.
The Tokyo Metropolitan Police Department stepped up security at facilities such as the Diet building, the Prime Minister's Official Residence and the U.S. Embassy last March after the military operation against Iraq began. Security levels were further heightened beginning last month, when Japan began the dispatch of its core Ground Self-Defense Force unit to Iraq.
MPD officials said it was regrettable that security at the embassy compound had been breached, and promised to review current operations to prevent a recurrence.