It was not known if Shirdon was in his office at the time of the explosion, CNN reported.
Unconfirmed reports indicate up to six people died in the attack in Mogadishu, Shabelle Media Network reported, although spokesman Garad Salad could not say whether there were any casualties.
Witnesses said the blast was strong enough to shatter windows in nearby buildings. The witnesses said a security guard died while trying to fight the attacker.
No group has taken responsibility for the bombing.