"Morsi was struggling on the floor in the courtroom for 20 minutes. Authorities unfortunately did not intervene to save him," Erdogan said during a televised speech in Istanbul, according to AFP.
"Morsi was killed, he did not die of natural causes," added the Turkish president.
Erdogan forged close ties with Morsi, Egypt's first civilian president and a prominent Muslim Brotherhood member.
He later condemned the military intervention in 2013 that toppled Morsi as a “coup” and an enemy of democracy. Egypt’s foreign ministry responded to Erdogan’s criticism at the time by summoning Turkey’s ambassador to Cairo in protest against “Ankara’s interference in Egyptian affairs.”
Erdogan was also critical of the Egyptian judiciary for sentencing Morsi to death in one of the many trials he faced after his ouster.
On Wednesday, the Egyptian president said he would follow up on the process related to Morsi's death.
"We will do whatever is needed for Egypt to be tried in international courts," he declared.
Morsi died on Monday at the age of 67 after collapsing in a Cairo court while on trial on espionage charges.
The attorney general's office in Egypt has said that Morsi was "transported immediately to the hospital", where medics pronounced him dead -- a version confirmed by a judicial source.
Morsi was buried on Tuesday, as rights groups including Amnesty International and Human Rights Watch called for an independent probe into the causes of his death.