A police vehicle was the real target of an attack that killed three workers in North Sinai on Monday, army spokesperson Colonel Ahmed Ali has said.
At least three people were killed and 17 wounded in Al-Arish in the early hours of Monday when suspected militants fired rocket-propelled grenades hitting a bus carrying workers to cement factory.
Ali extended his condolences to the families of the victims.
Egypt has witnessed a sharp rise in militant attacks in the Sinai Peninsula since former president Mohamed Morsi was removed by the army on 3 July amid mass protests against his rule.
At least 10 policemen died in militant attacks since Morsi's ouster.
Hardline Islamist groups based in North Sinai have intensified attacks on police and soldiers over the past two years, exploiting a security and political vacuum following the 2011 uprising that ousted autocratic president Hosni Mubarak.