The people who initially came into contact with the letters showed no symptoms of exposure to the poison, but three officers who later examined the New York letter experienced minor symptoms that have since abated, police said.
Bloomberg didn't comment on Thursday. On Wednesday, he said he didn't know why they were sent.
One of the letters "obviously referred to our anti-gun efforts, but there's 12,000 people that 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," said Bloomberg, adding that he didn't feel threatened.
The letters were the latest in a string of toxin-laced missives, but authorities would not say whether the letters to Bloomberg and Obama were believed to be linked to any other recent case.
In Washington state, a 37-year-old was charged last week with threatening to kill a federal judge in a letter that contained ricin.
About a month earlier, letters containing the substance were addressed to Obama, a U.S. senator and a Mississippi judge. One of the letters postmarked in Memphis, Tenn., was traced back to Tupelo, Miss., and a Mississippi man was arrested.
According to the Centers for Disease Control and Prevention, ricin is a poison found naturally in castor beans. Symptoms can include difficulty breathing, vomiting and redness on the skin depending on how the affected person comes into contact with the poison.
Bloomberg and Boston Mayor Thomas Menino founded Mayors Against Illegal Guns, which now counts more than 700 mayors nationwide as members. It lobbies federal and state lawmakers, and it aired a spate of television ads this year urging Congress to expand background checks and pass other gun-control measures after the school shooting in Newtown, Conn. The background check proposal failed in a Senate vote in April, and other measures gun-control advocates wanted -- including a ban on sales of military-style assault weapons -- have stalled.
Separately, Bloomberg also has made political donations to candidates who share his desire for tougher gun restrictions. His super PAC, Independence USA, put $2.2 million into a Democratic primary this winter for a congressional seat in Illinois, for example. Bloomberg's choice, former state lawmaker Robin Kelly, won.