(CNN) -- The letter-writer who sent threatening and possibly poison-laced notes this month to New York Mayor Michael Bloomberg and an anti-illegal firearm group said anyone who tried to seize the writer's guns would be "shot in the face," a source with knowledge of the letters said Thursday.
Preliminary tests indicate ricin was found in the letters, New York deputy police commissioner Paul Browne said Wednesday.
One of the letters addressed to the mayor's office was opened at the city government's mail facility. The second letter to the mayor was opened by Mark Glaze, director of Mayors Against Illegal Guns -- founded and co-chaired by Bloomberg -- in Washington on Sunday.
"You will have to kill me and my family before you get my guns," both letters say, according to the source. "Anyone wants to come to my house will get shot in the face. The right to bear arms is my constitutional God given right and I will exercise that right till the day I die."
Both letters were postmarked in Shreveport, Louisiana, on May 20, the American Postal Workers Union said on its website.
The suspicious material found in the two letters was a "pink-orange oily substance," Browne said, adding that it was the second of two tests that showed what appeared to be trace amounts of ricin.
Bloomberg is an outspoken critic of current gun laws. In March, he said nationwide background checks on all gun sales would save lives.
He said the note "obviously, referred to our anti-gun efforts, but there's 12,000 people (who) are going to get killed this year with guns and 19,000 that are going to commit suicide with guns, and we're not going to walk away from those efforts."
The substance is being tested at the National Bioforensic Analysis Center in Maryland, with conclusive results expected by Friday.
Some of the emergency services workers who touched the letter Friday were examined after they showed minor intestinal symptoms of ricin exposure on Saturday, Browne said. The symptoms have since subsided. Civilian workers showed no symptoms, Browne said in a statement.
"We take a lot of security measures as you know," Bloomberg said. "The men and women that open the mail for example ... they are well trained."