The FBI found "very low concentrations" of ricin on letters sent this week to President Obama, New York Mayor Michael Bloomberg and a gun control group, a spokesman said Friday.
Mayors Against Illegal Guns was founded by Bloomberg.
FBI spokesman Christopher Allen said lab results showed small amounts of ricin on the three letters.
Meanwhile, in what appears to be a separate incident of possibly poisoned mailings in Washington state, the FBI confirmed the presence of ricin on a letter sent to Fairchild Air Force Base near Spokane, the agency said.
One front of the FBI investigation advanced Friday in Texas, where authorities searched the home of a man in New Boston in connection with the threatening letters to Obama and Bloomberg.
FBI agents interviewed the man after his wife contacted authorities, saying she had found suspicious containers in their New Boston home, two law enforcement officials with knowledge of the investigation told CNN. One of the two officials, who spoke on condition of anonymity, said there were questions about what the man's wife told authorities.
"We're looking at the credibility of the complainant and using due diligence," the official said.
Both sources told CNN that the wife told authorities she found a suspicious container and ricin research on their computer.
Word of the search in Texas comes a day after the FBI said its agents were looking for a letter sent to the CIA that they believe is similar to other letters, mailed from Spokane, Washington, that contained the deadly toxin.
In all, at least 10 ricin-laced or suspected ricin-laced letters have been sent to government officials in recent weeks, including Obama.