Pete Piringer, a spokesman for the district's Fire and EMS Department, said schools reported receiving the letters Thursday but "no students have been in danger at any point," The Washington Post reported.
The FBI collected the letters and was conducting tests. Bureau spokesman Andrew Ames said the powder did not appear to be dangerous but couldn't say what it was, the Post said.
The letters, mailed from Dallas, had typewritten address labels and a message containing "AL AQEDA-FBI," a Washington Regional Threat and Analysis Center alert indicated.
Katherine Schweit, a spokeswoman for the FBI's Washington field office, said there was no indication the suspicious letters were linked to al-Qaida leader Osama bin Laden's death during a raid on his compound in Pakistan by U.S. forces, The Washington Times reported.
Mayor Vincent Gray said he was puzzled about why someone would target the city's schools.
"This is a dastardly act," Gray said during a news conference Thursday. "This is the kind of thing that alarms people unnecessarily."