Use of white powder sent through the mail meanwhile recall the deadly 2001 anthrax mailings in Washington, which killed five people and injured 17, rattling the US public just days after al Qaeda militants took over passenger planes and flew them into the World Trade Center and the Pentagon on September 11 that year. — File Photo

WASHINGTON: US authorities are investigating some 29 letters that have been packed with suspicious white powder and delivered to Washington-area schools, local media reported late Thursday.

The powdered material is, however, thought to be harmless cornstarch, said FOX 5 News, citing an official source.

The FBI said at a press conference that there had been no injuries related to the incidents.

The letters, which apparently originated in Texas, referenced the extremist group al Qaeda but did not a specific threat, said Fox 5.

The killing of al Qaeda leader Osama bin Laden on Sunday at the hands of US commandos in Pakistan has prompted authorities to urge caution for possible retaliatory strikes on US soil.

Use of white powder sent through the mail meanwhile recall the deadly 2001 anthrax mailings in Washington, which killed five people and injured 17, rattling the US public just days after al Qaeda militants took over passenger planes and flew them into the World Trade Center and the Pentagon on September 11 that year.

In that case a federal investigation concluded scientist Bruce Ivins, who killed himself in 2008, was to blame for the mailings.