An FBI intelligence bulletin obtained by The Associated Press said the two letters were postmarked Memphis, Tenn.
Both letters said: "To see a wrong and not expose it, is to become a silent partner to its continuance." Both were signed, "I am KC and I approve this message."
The letters had Washington on edge in the days after the Boston Marathon bombing. As authorities scurried to investigate three questionable packages discovered in Senate office buildings Wednesday, reports of suspicious items also came in from at least three senators' offices in their home states. The items were found to be harmless.
In addition, a Mississippi state lawmaker, Democratic Rep. Steve Holland of Plantersville, said Wednesday night that his 80-year-old mother, Lee County Justice Court Judge Sadie Holland, received a threatening letter last week with a substance that has been sent to a lab for testing. He said this letter was also signed "K.C."
"She opened it herself" on April 11 and told Holland about it three days later, Holland said.
He said she had not been to the doctor, but he planned to take her Thursday.
"She's fine," Holland said. "She's had no symptoms."
Neighbors, who said Curtis kept to himself and did not seem violent, were concerned by the idea that someone was making poison in the house that sits so close to their bedrooms and front yards.
Next-door neighbor Kayla Latch, 18, lives with her mother and her two brothers and said they were worried that toxic chemicals could be released when investigators enter the home.
"I'm afraid they might open it up and it (poison) might go into the air and hurt someone," Latch said.
Latch said Curtis lived with a woman and a teenage boy when he first moved in, but they appeared to have moved out.