An envelope containing suspicious white powder and threats against Americans was delivered to the US embassy in Malaysia, prompting health checks on three staff, the embassy and police said on Monday.
The staff - including the second secretary - were briefly quarantined for detoxification, police said. The embassy had remained open.
"Initial police investigations showed the powder was not anthrax," Prime Minister Abdullah Ahmad Badawi told reporters.
Police said they would not know for sure if it was a hoax until the powder had been tested.
A leaflet included in the envelope and purporting to come from a previously unknown group threatened to blow up the embassy and kill Americans in Malaysia, police said.
The leaflet, from a group calling itself Jemaah Muhajirin Mohamad, warned Washington to "take your army out of Iraq and remove the sanctions on Sudan or face the consequences", senior assistant police commissioner Aziz Bulat told Reuters.
"We don't think this Jemaah Muhajirin is a serious group, but we will carry out a thorough investigation," Aziz said.
The US government has feared anthrax attacks since five people died when letters with the deadly bacteria were sent to two US senators and several media groups in the United States in late 2001.