Starehe police chief Samuel Anampiu says the 40 were part of 75 patients who broke out from one ward, after forcing the door open early on Sunday morning.
“It looks from the initial investigations that they planned to escape for reasons we are yet to establish,” the divisional police chief said.
The remaining 35 were detained after they failed to leave the hospital’s compound.
The hospital’s administration reported the matter to the Muthaiga Police Station on Sunday evening and a manhunt has been launched.
“We have all their particulars and including their pictures and that will make it easy for us to identify them,” Anampiu said.
All the escapees were male patients.