He said that officials at one of the schools, having heard about incidents elsewhere, saw a similar letter in the incoming mail but did not bring it inside the building.
D.C. officials said Thursday evening that the schools that received the letters will open on time Friday.
The FBI was collecting letters from the scenes, according to spokesman Andrew Ames. Ames also said that the powder did not appear to be dangerous but could not say what it was.
The envelopes, which were mailed from Dallas, had typewritten labels with the addresses of each school and a letter containing the words “AL AQEDA-FBI,” according to an alert the Washington Regional Threat and Analysis Center sent to D.C. agencies.
The letters are identical to ones D.C. schools received in October 2010, the center said.
The letter sent to M.C. Terrell Elementary School in Southeast Washington appears to have been mailed Monday afternoon, according to a photo of the envelope appearing in the alert.
At a Northwest Washington school, the suspicious letter arrived in a white business envelope that contained a single sheet of typing paper and was delivered to the school along with other Postal Service mail, said an administrator.
Another letter, sent to School Without Walls in Northwest Washington, also arrived in the U.S. mail yesterday, said Linwood Jolly, the school’s PTA president, who was in a meeting with Principal Richard Trogisch when the letter was opened about 2:15 p.m.
Jolly said the letter contained about a pill capsule’s worth of white powder.
Staff writer Clarence Williams contributed to this report.