Five of the men who launched an attack in the heart of Iran's capital that killed at least 13 people previously fought for Islamic State, the country's intelligence ministry has said.
Police have increased their patrols in the streets and subway stations of Tehran following the attacks on Iran's parliament and the tomb of its revolutionary leader.
More than 40 others were wounded in the attacks which have been claimed by the Islamic State.
Commuters in the Iranian capital noticed more police than usual on the streets as dawn broke.
The move came after Mohammad Hossein Zolfaghari, a deputy interior minister, told state TV that "law enforcement activities may increase", adding that "we are focused" on intelligence gathering.
The attacks on Wednesday as politicians held a session in parliament and at the shrine of Ayatollah Ruhollah Khomeini shocked Iranians who so far had avoided the bloodshed that has followed the Islamic State group's rise in Syria and Iraq.
Iranian forces are backing embattled Syrian President Bashar Assad while the Shiite power also is supporting militias fighting against the extremists in Iraq.
The bloodshed shocked the country and came as emboldened Sunni Arab states - backed by US President Donald Trump - are hardening their stance against Shiite-ruled Iran.
The White House released a statement from Mr Trump condemning the terrorist attacks in Tehran and offering condolences, but also implying that Iran is itself a sponsor of terrorism.
"We grieve and pray for the innocent victims of the terrorist attacks in Iran, and for the Iranian people, who are going through such challenging times," the statement said. "We underscore that states that sponsor terrorism risk falling victim to the evil they promote."
The comments sparked anger from Iranians on social media, who recalled the vigils in Tehran that followed the September 11 attacks.
Foreign minister Mohammad Javad Zarif in a tweet on Thursday called the White House comments "repugnant" and accused the US of supporting terror.
"Iranian people reject such US claims of friendship," he tweeted.
Iran's powerful Revolutionary Guard indirectly blamed Saudi Arabia for the attacks.
A statement issued on Wednesday evening stopped short of alleging direct Saudi involvement but called it "meaningful" that the attacks followed Mr Trump's visit to Saudi Arabia, where he strongly asserted Washington's support for Riyadh.
The statement said Saudi Arabia "constantly supports" terrorists including the Islamic State group, adding that the IS claim of responsibility "reveals (Saudi Arabia's) hand in this barbaric action".
The "spilled blood of the innocent will not remain unavenged", the Revolutionary Guard statement said.
Emirati minister of state for foreign affairs Anwar Gargash said "the Iranian government should not use the attack in a very polarised situation against Saudi Arabia or claim that Saudi Arabia is somehow linked to the attack, because it isn't".
Ayatollah Ali Khamenei, the country's supreme leader, used the attacks to defend Tehran's involvement in wars abroad. He told a group of students that if "Iran had not resisted" it would have faced even more troubles.
"The Iranian nation will go forward," he added.
The violence began in mid-morning when assailants with Kalashnikov rifles and explosives stormed the parliament complex where a legislative session had been in progress.
The siege lasted for hours, and one of the attackers blew himself up inside, according to Iran's state TV.
Images circulating in Iranian media showed gunmen held rifles near the windows of the complex. One showed a toddler being handed through a first-floor window to safety outside as an armed man looked on.
As the parliament attack unfolded, gunmen and suicide bombers also struck outside Khomeini's mausoleum on Tehran's southern outskirts.
Khomeini led the 1979 Islamic Revolution that toppled the Western-backed shah to become Iran's first supreme leader until his death in 1989.
Iran's state broadcaster said a security guard was killed at the tomb and that one of the attackers was killed by security guards. A woman was also arrested. The revered shrine was not damaged.
Police on Thursday said they now held six suspects as part of their investigation into the attacks.
Reza Seifollahi, an official in the country's Supreme National Security Council, was quoted by Iranian media as saying that the perpetrators of the attacks were Iranian nationals.