Egypt's interior minister has accused the Muslim Brotherhood and Palestinian group Hamas of planning the assassination of the country's chief state prosecutor in last year's bombing.
Magdy Abdel-Ghaffar said during a televised address that Hamas, which controls Gaza, trained the Muslim Brotherhood for the operation that killed the 65-year-old Hisham Barakat in June 2015.
He said: "Hamas trained, prepared, and oversaw the implementation, of the assassination of a senior Egyptian official, the first in 25 years."
Hamas spokesman Sami Abu Zuhri denied the accusations, saying they were "baseless and are not in harmony with the efforts being exerted to develop the relationship between Hamas and Cairo".
A car bomb last June killed Mr Barakat, who oversaw thousands of cases against Islamists.
Mr Abdel-Ghaffar said dozens of people had been arrested in relation to the bombing.
Earlier in the day, the country's current chief prosecutor, Nabil Sadek, said in a statement that his office had ordered the arrest of six people to remain in custody for 15 days until the start of the investigation.
Most of the people arrested are believed to be students at the Al-Azhar University in Cairo, which is a Sunni Muslim institution and Egypt's oldest degree-granting university.
At the time, there were no reliable claims of responsibility for the attack and the Muslim Brotherhood denied any involvement.
But the Islamic group has been held responsible by the Egyptian government for most political violence in the country over recent years.
Shortly after the attack, the government in Cairo voted for anti-terrorism laws that broadened the definition of terrorism, gave police greater powers of arrest and tightened restrictions on free speech.
Additional reporting by the Associated Press
