"This attack was against Turkey," Erdogan told reporters, adding that "we do not differentiate between the deaths." "The target of this attack was our citizens (in Ankara) as well as the totality of our country."
The Turkish leader was referring to explosions that rocked a train station in Ankara, killing 97 people and injuring about 246 more.
Afterward, Selahattin Demirtas -- leader of the People's Democratic Party, a pro-Kurdish group among those involved in Saturday's peace rally-turned-bloodbath -- questioned whether there was sufficient security for such opposition demonstrations.
On Tuesday, Erdogan -- while not offering specifics -- did acknowledge, "Of course, there was a mistake.
"... But its size will become clear after the investigations," he said.
Erdogan then talked about Turkey's fight over the past 40 years with different groups, who all aimed to unsettle the country's "unity, peace (and) stability."
"This is not the first time Turkey is (battling) terror organizations (or) terror's insidious and cruel methods," he said.
Turkish official: Investigators focusing on ISIS
While Erdogan spoke about the attacks, Prime Minister Ahmet Davutoglu paid tribute by laying flowers at the attack site Tuesday.
The day before, he told Turkish television station NTV that the prime suspect is ISIS, the Islamist terror group that has captured vast swaths of neighboring Syria and Iraq. The Turkish government has stepped up its own efforts against the group -- even though Kurdish groups that Erdogan's ruling party has long opposed are on the same side of the battle -- including allowing the United States to launch strikes
on the militant group's positions from Incirlik Air Base in southern Turkey.
The Kurdistan Workers' Party, PKK, and the leftist DHKP-C
could also be behind the blasts, the prime minister added.
Authorities haven't named any suspects, though Deputy Prime Minister Numan Kurtulmus has said "a large number of people have been detained," according to the semiofficial Anadolu news agency
. He noted that the bombers used five kilograms each of TNT.
Erdogan said authorities are conducting DNA tests, checking on intelligence and piecing together evidence to get to the bottom of what happened and who was responsible.
"All of our institutions are working on this, and the results of this work will show which organizations did this," he said. "... But we must be a little patient. ... I believe we will, together, find out who conducted this heinous act."
Comes weeks ahead of election
The blasts rocked crowds at a lunchtime peace rally calling for an end to the renewed conflict between the PKK and Turkish government helmed by Erdogan.
It came not only as Turkey dealt with the ISIS threat simmering on its southern border, but also three weeks ahead of an election. Davutoglu told NTV the attack was obviously intended to have an impact on those contests.
CNN's Tim Lister writes
that whoever chose the Ankara rally as a target wants to stoke polarization and violence in Turkey -- and destroy an already fragile political dialogue.
The blast has already affected the election. Erdogan's Justice and Development Party has declared it won't hold any of its scheduled election rallies until Friday and that subsequent rallies would be focused on peace and anti-terror themes, Anadolu reported .
The main opposition group, the Republican People's Party, has also said it would not hold any rallies until October 14, according to the English-language Hurriyet newspaper.
In an interview with CNN's Fred Pleitgen on Monday, Demirtas -- whose party lost two parliamentary candidates in the blast -- said any large-scale events may not be wise at this point.
"We are of the opinion that it'd be wiser not to hold large rallies in open spaces," he said. "We cannot continue our election campaigns as if nothing happened. It wouldn't be right politically or ethically."