An exercise buff caused a ruckus at a San Jose, Calif., post office when he showed up to mail a package donning a mask and a weight-packed vest, gear he says is part of a so-called "CrossFit" workout.

Long Hoang, 29, was at the post office Tuesday simply to mail a package of calendars that had been sent to him by mistake. Struggling to jam the package -- which just barely fit into the mailbox -- down the chute, Hoang had no idea he was the subject of calls to authorities about a suspicious man who ran up to the mailbox wearing what looked like a gas mask and a bulletproof vest. After he managed to get the box down the chute, Hoang jogged away, continuing on with his workout.

A bomb squad and firefighters showed up. Authorities closed down nearby streets, reported CBS 5 in San Francisco. Investigators, using a robot, detonated the box of calendars Hoang had been trying to send to its rightful recipient.

When the story hit local news outlets, Hoang started receiving phone calls, The San Jose Mercury News reported.

"My friends kept messaging me, Is this you? Is this you?" Hoang, a nursing student who lives near the post office, said. He called police Tuesday evening to clear up the confusion.

"The guy said he was wearing a cardio mask," Sgt. Jason Dwyer told The Mercury News. "It was his cardio day, and he was trying to lose weight."

Hoang said he has been following the CrossFit exercise regimen for about there months. The elongated mask he wears over his nose and mouth makes breathing difficult, to simulate high-altitude training; his black vest holds 10 pounds of weights.

The workout is "this really creative combination of weight lifting, gymnastics and rowing," Hoang told The Mercury News. "It was like straight out of a movie ... Some of my friends are telling me, 'Hello? 9/11? Anthrax? Blah. Blah. Blah.' And I'm just thinking about my finals and staying in my own little zone."

Watch an interview with Hoang about the misunderstanding on San Francisco's CBS 5 website.