The man has been identified as Long Hoang, a 29-year-old San Jose resident and student at Cal State East Bay in Hayward, police said.
On Tuesday afternoon, Hoang, wearing a weighted vest and a special mask, ran up to a mail collection box near a post office at 1750 Lundy Ave., dropped a package into it and ran away.
A customer leaving the post office saw a man wearing what appeared to be a gas mask and what looked like a bulletproof vest, and reported him to authorities, prompting a police response and four-hour lockdown.
Customers and workers were told to leave the building while a robot and an officer wearing a protective blast suit checked the mailboxes.
Police found the suspicious package and determined that it was harmless. Dwyer said it was a stack of calendars Hoang had intended to mail during his jog.
Dwyer said Hoang was wearing the mask and vest as part of a CrossFit exercise regimen. The mask has the effect of mimicking high-altitude training, and the vest had small weights attached, which helps increase endurance.
"His story matched up perfectly," Dwyer said today.