( CNN ) -- The suspicious powder inside a swastika-adorned package sent to an Arizona congressman is `` nontoxic , '' an FBI spokesman said Friday . Staffers checking mail in the Tucson , Arizona , office of U.S. Rep. Raul Grijalva on Thursday found a white , powdery substance and drawings of two swastikas inside an envelope , Grijalva campaign spokesman Adam Sarvana told CNN . Almost a dozen people were in the office when the incident happened around 12:30 p.m. Thursday ( 3:30 p.m . ET ) , he said . All of them were checked on-scene by local authorities and sent home . Scientists at an FBI laboratory in Phoenix , Arizona , conducted a full analysis of the substance , according to Sarvana . Those tests came back negative midday Friday , according to FBI Special Agent Manuel Johnson . Johnson said the FBI would have no further comments , as `` the investigation is ongoing . '' U.S. Capitol Police are also participating in the investigation , according to Sgt Kimberly Schneider , a spokeswoman for the force . The incident was the third security issue at Grijalva 's district offices this year , Sarvana said . `` It 's just a continuation of a problem we 've been having , '' Grijalva told CNN affiliate KVOA . In April , the congressman closed two local offices for an afternoon after receiving threats regarding his position on controversial immigration legislation . And in July , a bullet shattered a window in his Yuma , Arizona , office , KVOA reported . `` I do feel a great deal of concern for the people that work with me , for me . I feel concerned for the constituents that come to that office , '' Grijalva told KVOA Thursday . `` Obviously , we 're going to have to make more changes . '' CNN 's Melanie Whitley and Carol Cratty contributed to this report .
NEW : U.S. Capitol Police are working with the FBI on the investigation . Comment: nontoxic ' A campaign spokesman says there were two swastikas on the package . It is the third security issue at Grijalva 's district offices this year , a spokesman says .