The first "suspicious" envelope was reported in Oklahoma City on Monday, and more than three dozen similar envelopes have since been received in 10 states, FBI Special Agent Richard Kolko said.
The envelopes are targeting Chase banks and branches of the federal Office of Thrift Supervision, he said. Additionally, one Federal Deposit Insurance Corporation office received a similar envelope.
The envelope mailed to Daly City arrived at the Office of Thrift Supervision West Region at 2001 Junipero Serra Blvd. on Tuesday.
Kolko said it "appears to be tied to the mass mailings" and that the letter contained a threat similar to that received by institutions across the country.
Field tests of the suspicious powder in Daly City and other cities have come back negative for anything dangerous, according to Kolko, who added the powders will be tested further at regional laboratories.
Daly City police said Tuesday a hazardous materials team took about four hours to determine that the material was calcium carbonate and harmless.
Kolko said that even if the letters are a hoax, they are no joke.
"It has a major impact on commerce," Kolko said. "It terrorizes people. It takes law enforcement out of the regular duties."
The FBI, U.S. Postal Inspection Service and local law enforcement agencies are continuing to investigate the case, and no arrests have been made.
As of this morning, the Daly City incident was the only one in California believed to be related to the nationwide mailings, Kolko said.
Bay Area residents with information on the mailings are asked to call the FBI at (415) 553-7400 or their local law enforcement agencies.