The FBI is investigating a series of letters containing a white powdery substance sent to government buildings in L.A. County.
Few details about the letters have been released, but FBI spokeswoman Laura Eimiller said they appeared to be addressed to specific officials.
Investigators are still testing the substance contained in the letters to determine what it is.
Those who received and handled the letters did not show symptoms of illness, Eimiller said. The letters did not include specific threats, just the white substance.
The news comes after at least two other incidents involving letters were reported this week.
Suspicious letters containing a white powdery substance were sent to personnel Thursday at a downtown Los Angeles courthouse, according to the Los Angeles County Sheriff's Department.
A hazardous materials team and FBI investigators responded to the Stanley Mosk Superior Court in the 100 block of South Hill Street and tested the substance.
On Wednesday, a mail room worker at the Van Nuys courthouse discovered white powder on an envelope addressed to Superior Court Judge Leland B. Harris, Los Angeles police said.
Police said the substance appeared to be boric acid, a common pesticide used to kill insects and weeds. Skin contact may not be harmful, but in the form of borax it poses a greater danger, especially if swallowed.
The FBI is looking at any possible connections among letters that have been received. Eimiller said investigators noted the timing of the letters and other similarities.
No one has been arrested, but investigators are looking for the sender or senders.
The FBI is working with the Los Angeles police and fire departments, Burbank police and fire departments, the county health department, the Sheriff's Department and the U.S. Postal Service.