There was no reason to believe they were of a potential danger. Gin was fairly new but a child prodigy, with no proof of connection to what was happening. Aizen and Tosen had no interaction with what happened either as far anyone knew. There also is the issue that no one knew what was happening at the time. They just weren't likely suspects even to the Visored at the time.
Shunsui did actually suspect something was up with Aizen, but Aizen had many, many witnesses to act as alibis for him.
Gin was new to the Gotei 13 and was considered a genius. Kaname was known to fight for "justice' and was always loyal. Aizen was most suspicious, but covered his tracks with Kyoka Suigetsu (i.e. using a body double).