Iran’s Supreme Leader Ayatollah Ali Khamenei said there would be a "definitive punishment of the perpetrators and those who ordered it."
The mailer, picturing Iranian soldiers, clerics and government officials, says, "We live in the most dangerous moment in a generation.”
Judiciary spokesman Gholamhossein Esmaili did not say how many individuals have been detained or name them.
Arizona Rep. Paul Gosar tweeted a doctored image of Barack Obama with Iranian President Hassan Rouhani and got testy when people pointed out it was fake.
The move is Iran's latest step away from its nuclear deal with world powers since Trump withdrew from the accord over a year ago.
His speech at the U.N. General Assembly came shortly after U.S. Secretary of State Mike Pompeo announced new U.S. sanctions targeting Iran’s ability to sell its oil.
A centerpiece of this year's U.N. schedule will be a Monday session on climate change that Trump plans to skip.
Iran’s president Hassan Rouhani separately promised to unveil a regional peace plan at this week's upcoming high-level meetings at the United Nations.
Ayatollah Ali Khamenei's comments follow reports about a possible meeting between President Donald Trump and his Iranian counterpart, Hassan Rouhani.
“Americans have to realize that warmongering and warmongers are not to their benefit,” the Iranian president said in televised remarks.