A homemade bomb packed with nuts and bolts exploded early Monday at a checkpoint in north Cairo, injuring three policemen, local media outlets reported.

Security sources said that the blast occurred in Hadayek al-Kobba, and could be heard in nearby neighborhoods.

A bomb disposal expert said that the bomb contained large quantities of nuts and bolts, according to website of the state-owned Al-Ahram newspaper.

The bombing is the third reported in one week to target security forces. Police and military have come under repeated attack throughout the capital and other parts of Egypt since the July ouster of former President Mohamed Morsi.

Two police officers were killed Thursday, one in Ismailia and another north of Cairo, in Qalyubiya.

The death in Ismailia occurred during an operation to arrest suspects in the November 14 murder of Lieutenant Colonel Mohamed Mabrouk, according to an Interior Ministry statement. 

Two days earlier, on Tuesday, Ansar Beit al-Maqdes, a Sinai-based jihadist group, claimed responsibility for the Mabrouk killing. Mabrouk was reportedly involved in several high-profile cases, including an allegation of espionage against ousted President Mohamed Morsi.

The search for the suspected assailants led to an exchange of gunfire with what the ministry described as “jihadi elements.”

On Wednesday, 11 soldiers were killed in northern Sinai when a car bomb exploded. On the same day, a police checkpoint in east Cairo was bombed, injuring four policemen.