The Secret Service agents, including some supervisors, were suspended without pay over security lapses at last year’s Butler campaign rally, according to an official.

The Secret Service said on Thursday that it was suspending six agents involved in securing the site of a campaign rally where a gunman tried to assassinate Donald J. Trump last summer.

Almost immediately after the assassination attempt on Trump last July, lawmakers of both parties questioned the security failures that allowed a 20-year-old lone gunman the opportunity to target and ...