The man suspected of killing British lawmaker Jo Cox was a longtime supporter of a U.S.-based. neo-Nazi organization, according to a watchdog group that tracks extremists, reports ABC News.

Cox, a British Member of Parliament (MP) and mother of two, was killed in a shooting and stabbing midday Thursday after a town hall meeting in West Yorkshire, England.

She was an MP for the Labour Party. Cox, 41, was campaigning for the United Kingdom to remain in the European Union and was known for her work on Syria-related issues. She reportedly had been subjected to hate mail and police were poised to put extra security in place for her.

According to records obtained by the Southern Poverty Law Center, the U.S.-based organization that monitors hate groups, the suspect, whom the center identified as Thomas Mair, was a "dedicated supporter" of the National Alliance, a once-prominent white supremacist organization in the United States.

He purchased a manual from the National Alliance that included instructions on how to build a pistol, the center said, as well as other publications.

Police have said they arrested a 52-year-old suspect and officials confirmed to ABC News today that they were not looking for anyone else. Police have not released the suspect's name and his motives remain unknown.