Nearly 30 schools in Washington DC have received envelopes containing a mysterious white powder and with ‘AL AQEDA-FBI’ written on them, the Federal Bureau of Investigation said on Thursday. Preliminary testing has proved that the powder was not harmful, the FBI has said.

Though recent incidents of letters with mysterious powder have passed without harming anyone the modus operandi is powerful enough to spook people who witnessed the 2001 deadly anthrax mailing attacks. The anthrax attack had killed five people close on the heels of the 9/11 attacks by Al-Qaeda.

The Star-Telegram reported that stamps on the letters were cancelled on May 2, triggering speculation if the incident was prompted by the killing of terror mastermind Osama bin Laden in Pakistan.

The Washington Regional Threat and Analysis Center said in a release that at least six letters were postmarked from Dallas. They contain a white powder which is similar to the content of letters sent to schools in District of Columbia in October 2010.

The head of the FBI's Washington field office said addresses on the envelopes were typed out. The letters were addressed to the schools, not individuals. The report also quotes an official as saying that the white powder had the consistency of cornstarch.

Envelopes containing mysterious white powder had been sent to the governors of Alabama, Michigan, Minnesota, Mississippi, Missouri, Montana and Rhode Island in 2008. An FBI investigation found that the suspicious material was not hazardous.

Also in November 2008, a temple in Los Angeles, which was at the site of a gay rights protest, received a white powder envelope. A clerk, on whose hands the powder spilled while opening the letter had reported no signs of illness in that incident.