Herrmann said his backpack contained screws and nails, an apparent attempt to inflict further damage.
Officially this attack is not confirmed as terrorism, he said, but there are strong indications it may be.
Special Forces in Munich are on the scene investigating, police said.
Violence has struck Germany in several places recently.
On Sunday, a man killed a woman with a machete in Reutlingen. The 21-year-old Syrian asylum seeker came to Germany one year ago, according to a police statement, and he was known to police for property thefts and assault.
Police said the woman was 45 years old and from Poland.
On Friday, a man in Munich went on a shooting spree in a busy shopping district, killing nine people before killing himself
, authorities said.
Authorities have not found a link to terror groups. Police said he may have planned the attack for a year.