An envelope of “white, powdery substance” resulted in the temporary quarantine of more than 50 people the Beloit Area Community Health Center Wednesday, and 15 people were transported to nearby hospitals after experiencing burning sensations in their eyes and noses, multiple news sources reported Wednesday.

The envelope, which was reportedly dropped at the center by a man in his 50s with a thin build and grey hair and beard, was originally determined not to be a “biological substance” by officials but is currently undergoing further tests to determine what it was, Beth Jacobsen, assistant to the city manager of Beloit, told the Rockford Register Star.

Beloit police Capt. Vince Sciame told the Register Star that investigators believe the suspect may have been a patient at the center. He said police would be investigating his whereabouts.

“There are no serious complications at this point,” Sciame told the Milwaukee Journal Sentinel Wednesday.