NBC Washington obtained a picture of one of the suspicious letters sent to local schools last week.
Another suspicious letter was found at a school in the District Thursday. That raises the total number of letters discovered since last week to 40.
Thursday's letter arrived at Fletcher-Johnson Education Center in Southeast. D.C. police said it's similar to the other ones found at schools and post offices last week.
All those letters were postmarked from Dallas, Texas. Investigators said they contained a harmless white powder and a note referencing al-Qaida.
Police said the FBI has possession of Thursday's suspicious letter. It was sent to Dallas as part of the continuing investigation.