Egyptian security officials say a bomb blast in downtown Cairo near the Foreign Ministry has killed two police lieutenant colonels and a recruit.
Authorities say several other police officers were wounded in the explosion that targeted a police checkpoint Sunday.
There was no immediate claim of responsibility, but the operation resembled those carried out by Islamist insurgents seeking to topple the U.S.-backed government, underlining security challenges facing President Abdel Fattah el-Sissi.
Militants have killed scores of police in bombings and shootings since Egypt's military toppled the country's first democratically elected civilan president Mohamed Morsi last year.
Authorities have killed more than 1,400 of Morsi supporters since his ouster and jailed thousands more.
One of the police officers killed in Sunday's blast, Lieutenant Colonel Mohamed Mahmoud Abu Sareeaa, was a critical witness in a trial of Morsi related to a 2011 mass prison break, court and security sources told Reuters.
It was not clear if he was targeted or just happened to be at the site of the explosion.
Human Rights Watch (HRW) said last month Sissi should be investigated for possible human rights violations in connection with the killing of hundreds of protesters last year in Cairo.
Military Helicopter Crashes, 6 Killed, 1 Hurt
Meanwhile, Egyptian state television said a military helicopter crashed while on a training mission southwest of Cairo, killing six and wounding one.
The aircraft crashed on Sunday in Fayoum, an oasis province, about 100 kilometers (62 miles) south of Cairo, an army statement said.
Defense Minister General Sedki Sobhi has ordered an investigation into the incident.
"During military training, a technical failure led to the crash of an aircraft carrying troops in Kom Aushim in Fayoum province ... killing six soldiers and injuring one," the army said.
No further details about the accident were immediately available.
Some material for this report came from Reuters, AP and AFP.