A letter addressed to President Obama believed to contain the deadly poison ricin closely resembles two letters sent to New York City Mayor Michael Bloomberg and his pro-gun control advocacy group.

The Secret Service on Thursday intercepted the letter to the president at the White House’s mail screening facility.

The New York City Police Department said Wednesday that the letters to Bloomberg and Mark Glaze, director of the gun control group Mayors Against Illegal Guns, referenced the recent debate over the country’s gun laws.

The FBI has taken the lead on investigating the letter to Obama and is testing the letter for ricin. If it tests positive, it would be the second parcel containing ricin that has been addressed to the president since last month.

NBC News late Thursday reported that authorities are questioning a person of interest in connection with the letters.

The former Army veteran now works for the Defense Department, according to the network, which quoted sources who said he is not considered a suspect at this time.

Federal prosecutors have charged James Dutschke in the earlier incident with sending Obama and Sen. Roger WickerRoger Frederick WickerHillicon Valley: Zuckerberg to meet with lawmakers | Big tech defends efforts against online extremism | Trump attends secretive Silicon Valley fundraiser | Omar urges Twitter to take action against Trump tweet Tech giants defend efforts against extremist content The 13 Republicans needed to pass gun-control legislation MORE (R-Miss.) letters containing the poisonous substance, which is made from castor bean waste.

Dutschke was arrested after officials initially tried to bring charges against Paul Kevin Curtis, whom Dutschke was allegedly attempting to frame as part of an escalating feud between the two Mississippi men.

Bloomberg on Wednesday vowed that the letters would not deter his efforts to push lawmakers to enact new gun controls.

“The letter, 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," Bloomberg said, according to CNN.

Reports said Glaze opened one of the letters Sunday in Washington and had shown no medical symptoms. The second letter sent to Bloomberg was opened by staffers at a government mail facility in New York.

Bloomberg’s group has been a forceful proponent of tougher gun ownership restrictions in the renewed debate over the nation’s gun laws after last December’s mass shooting at an elementary school in Connecticut.

The billionaire mayor has pledged to use his vast fortune to counter the efforts of the nation’s largest gun lobby, the National Rifle Association and to elect pro-gun control candidates in the 2014 midterms.

His group has run ads supporting a bipartisan compromise on background checks and criticizing lawmakers who have opposed the measure.

This week the group is launching a new campaign to pressure Sens. Jeff FlakeJeffrey (Jeff) Lane FlakeFlake donates to Democratic sheriff being challenged by Arpaio in Arizona The Hill's Morning Report - Trump says US-China trade talks to resume, hails potential trade with Japan, UK Joe Arpaio to run for Maricopa County sheriff in 2020  MORE (R-Ariz.), Kelly AyotteKelly Ann AyotteGOP fears Trump backlash in suburbs Trump makes rare trip to Clinton state, hoping to win back New Hampshire Key endorsements: A who's who in early states MORE (R-N.H.) and Max BaucusMax Sieben BaucusBottom line Overnight Defense: McCain honored in Capitol ceremony | Mattis extends border deployment | Trump to embark on four-country trip after midterms Congress gives McCain the highest honor MORE (D-Mont.) to back a bill that would prevent people on terror watch lists from purchasing weapons.

Gun control advocates are seeking to close the so-called “terror gap.” Under current law, people on the FBI’s “no fly” list are not included in the separate National Criminal Instant Background Check System used for prospective firearm purchasers.

—This report was first published at 12:20 p.m. and has been updated.