Joseph Nkarissery, who is in charge of security, also admitted that the response was poorly co-ordinated.
Militants from the Somali-based Islamist al-Shabab group killed 148 people during the day-long siege at the campus in the north-eastern town.
There has been much public criticism over the alleged security failings.
Last week, seven top policemen were suspended by Mr Nkarissery following an initial inquiry into the security failings.
Universities had posted memos warning students of possible violence and the principal of Garissa University College is reported to have requested in vain additional security at the campus.
Most of those who died in the raid were students.
The attackers singled out Christians to be killed and spared Muslims.
It was worst attack to date in Kenya by al-Shabab, which is affiliated to al-Qaeda.