At least two Egyptian policemen were killed in a bombing outside the foreign ministry in Cairo, the interior and health ministries said, revising an earlier toll.
Nine people were wounded in the blast that targeted a police checkpoint, assistant interior minister Abdel Fattah Othman told the official MENA news agency on Sunday.
One of the officers, Mohamed Mahmud Abu Sarie, had testified in a court case on a prison break involving ousted president Mohamed Morsi in 2011, when he was an opposition leader jailed by former strongman Hosni Mubarak, a security official said.
It was unclear whether Abu Sarie had been targeted over his role in the trial.
The armed group Ajnad Misr claimed the bombing, saying it was defending "the oppressed" and was in retaliation for a government crackdown on Morsi supporters.
The bombing targeted "the forces of the criminal security apparatuses to give them a taste of what they do to Muslims," the group said in a statement on Twitter.
"Vengeance operations being carried out by a blessed group from this proud people will not stop."
Militants have killed scores of policemen since the military toppled Morsi in July 2013.
In a separate incident later the same day, six Egyptian soldiers were killed when a military aircraft crashed while on a training mission south of Cairo.
"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 in a statement, without providing further details.
Armed groups have killed scores of policemen in bombings and shootings since Egypt's military toppled Islamist president Mohamed Morsi in July 2013.
Fighters have in the past set off several bombs in succession, to target first responders after the initial attack.
Two policemen, both bomb disposal experts, were killed trying to defuse bombs outside the presidential palace in June.