Sources close to the investigation said Muzaffer Tekin was being treated under police guard in a hospital on Istanbul's Asian side, where he was taken on Saturday with a minor knife injury in the chest.
Police believe that he may have attempted suicide.
The man, described by some media as the person who planned an attack on a court in Ankara on Wednesday in which a senior judge died and four others were wounded, was expected to be formally detained after being discharged.
Asked by the state-run Anatolia news agency whether he could confirm Tekin's hospitalisation, Celalettin Cerrah, the Istanbul police chief, said: "I was told so. I do not know myself. The development is still very fresh."
Some newspapers described Tekin as the "mentor" of the gunman who burst into a meeting room in the Council of State shouting "I am a soldier of Allah" and shot the judges to "punish" them for judgments confirming a ban on headscarves in public institutions and universities.
Other reports said he was a senior member of an illegal grouping that calls itself The Patriotic Force Action Team, to which the court assailant and several other people detained in the investigation belonged.
Thousands of Turks protested
against the attack on the court
The gunman, Alparslan Arslan, 29, a lawyer in Istanbul, and eight other suspects are being questioned by anti-terror police in Ankara.
The attack has heightened tensions in Turkey as the Islamist-rooted government of Recep Tayyip Erdogan faced accusations that its opposition to the headscarf ban and criticism of court rulings had emboldened extremists.
Turkey's most senior general on Friday publicly backed protests against the attacks on the court and urged for collective action to protect Turkey's secular character.
"Expecting and suggesting such protests and reactions for the future can never be the right attitude"
Recep Tayyip Erdogan,
Turkish prime minister
But on Saturday, Erdogan accused General Hilmi Ozkok of being irresponsible.
"Expecting and suggesting such protests and reactions for the future can never be the right attitude," the prime minister said.
"I should say this openly. Because we, as the people who are in positions which require responsibility, should know what we should advise and how."
General Ozkok had lauded the protests for which thousands turned out. "But this reaction should not be limited to a single day, to a single event. It must gain continuity and it should be followed by everyone all the time," Ozkok had said.