Iran said on Thursday that gunmen and bombers who attacked Tehran on Wednesday were Iranian members of ISIS who had fought in the militants' strongholds in Syria and Iraq – deepening the regional ramifications of the assaults.
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An Iranian journalist claimed on Twitter that a woman arrested by authorities was a mastermind behind the attacks.
Police, who said they were holding six suspects as part of their investigation into the attacks, also increased their patrols in the streets and subway stations of Tehran on Thursday.
The move comes a day after a pair of ISIS-claimed attacks on Iran's parliament and the tomb of its revolutionary leader killed at least 13 people and wounded over 40.
Mohammad Hossein Zolfaghari, a deputy Interior Minister, told state TV that "law enforcement activities may increase” and added that authorities are “focused on intelligence” gathering.
The bloodshed shocked the country and came as emboldened Sunni Arab states – backed by U.S. President Donald Trump – are hardening their stance against Shi'ite-ruled Iran.
The White House released a statement from 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. Iranian Foreign Minister Mohammad Javad Zarif in a tweet on Thursday called the White House comments "repugnant" and accused the U.S. of supporting terror.
"Iranian people reject such U.S. claims of friendship," Zarif tweeted.
The violence began in midmorning 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 slain by security guards. A woman was also arrested. The revered shrine was not damaged.
Iran's powerful Revolutionary Guard indirectly blamed Saudi Arabia for the attacks. A statement issued Wednesday evening stopped short of alleging direct Saudi involvement but called it "meaningful" that the attacks followed Trump's visit to Saudi Arabia, where he strongly asserted Washington's support for Riyadh.
The statement said Saudi Arabia "constantly supports" terrorists including ISIS, adding that the group's 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 told The Associated Press on Wednesday that "the Iranian government should not use the attack in a very polarized 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.