Sheriff's deputies say the powder, attached to brochures for nutritional supplements, was sodium cyanide.
Police arrested 50-year-old Kathryn Schoonover, a cancer patient who admitted to having a background in chemistry.
Federal attempted murder charges were expected to be filed Wednesday and prosecutors have asked that she be held without bond.
Investigators say they found a list of 100 people they believe were her intended victims. Many of them work in the medical profession or law enforcement and have had contact with her in the past.
The postal service has alerted every employee in the country of the possibility that Schoonover could have already been successful in sending out her poison mail.
"If she had done any previous mailings prior to yesterday, if it was in the mail stream, and someone comes in contact with cyanide poison, obviously it could be deadly," said Venice, California, postmaster Vince McCloskey.
Authorities confirm that four people from Schoonover's list already received similar letters, but they don't know if they contained poison. Natalie Mooney says she got one of the mailings.
"I didn't open it but the powder was coming right through the package and it smelled, it smelled awful," said Mooney.
The last national cyanide scare involved Tylenol in 1982. Seven people died after taking poisoned capsules.
In this case, the poison wasn't so well disguised. However, officials say the brochures were real and probably would have tricked someone into taking the deadly powder.
Reported by Sandra Hughes
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