• The United States obtained intelligence about an Iranian plot to assassinate former President Donald Trump in recent weeks.
• That intelligence led the U.S. Secret Service to increase security around Trump, who is the Republican presidential nominee, the officials said.
• The intelligence and boosted security occurred before Trump was targeted Saturday in an attempted assassination by Thomas Crooks at a campaign rally in Pennsylvania.

The Secret Service increased its protection for Donald Trump in recent weeks after U.S. officials obtained intelligence about an Iranian plot to assassinate the former president, three officials briefed on the matter told NBC News on Tuesday.

The intelligence was acquired and Trump's security heightened before the Republican presidential nominee was targeted in an attempted assassination by 20-year-old gunman Thomas Crooks at a campaign rally in Pennsylvania on Saturday.

CNN first reported the news of the Iranian plot, which raises more questions about the Secret Service's failure to prevent the shooting by Crooks from a nearby building's rooftop.

There are no known links between Crooks and the Iran plot.

"Upon learning of the increased threat, [the National Security Council] directly contacted [United States Secret Service] at a senior level to be absolutely sure they continued to track the latest reporting," a national security official told NBC News.

"USSS shared this information with the detail lead, and the Trump campaign was made aware of an evolving threat," the official said. "In response, Secret Service surged resources and assets for the protection of former President Trump. All of this was in advance of Saturday."

Anthony Guglielmi, the chief spokesman for the Secret Service, in a statement to CNBC, said, "The Secret Service and other agencies are constantly receiving new potential threat information and taking action to adjust resources as needed."

"We cannot comment on any specific threat stream other than to say that the Secret Service takes threats seriously and responds accordingly," Guglielmi said.

National Security Council spokeswoman Adrienne Watson in a statement to NBC News said that law enforcement authorities investigating the shooting by Crooks have "not identified ties between the shooter and any accomplice or co-conspirator, foreign or domestic."

"As we have said many times, we have been tracking Iranian threats against former Trump administration officials for years, dating back to the last administration," Watson said.

"These threats arise from Iran's desire to seek revenge for the killing of Qassem Soleimani.  We consider this a national and homeland security matter of the highest priority," she said.

Soleimani, who at the time was Iran's most powerful general, was killed in January 2020 by a U.S. airstrike in Baghdad, Iraq, when Trump was president.

Iran previously was known to be targeting other former top Trump administration officials who were involved in the operation that led to Soleimani's death, among them former Secretary of State Mike Pompeo and former National Security Advisor John Bolton, who continue to have security details as a result of those plots.

Iran's mission to the United States in a statement on the alleged plot said, "These accusations are unsubstantiated and malicious."

"Trump is a criminal who must be prosecuted and punished in a court of law for ordering the assassination of General Soleiman," the mission said, according to the Islamic Republic News Agency, Iran's state news service.

"Iran has chosen the legal path to bring him to justice," the mission said.

The Secret Service, which is responsible for protecting former presidents and major presidential candidates, has faced withering criticism for failing to stop Crooks from shooting at Trump and rally attendees.

One man was killed, and two others were seriously injured in the shooting, which occurred two days before Trump was formally nominated as the Republican Party's presidential candidate at the GOP convention in Milwaukee.

The agency has said that securing the building was the responsibility of local police in Butler Township because it fell outside the security "perimeter" of the rally site, which was the Secret Service's responsibility.

Secret Service Director Kimberly Cheatle told ABC News in an interview that the shooting "was unacceptable ... and it's something that shouldn't happen again."

"The buck stops with me," Cheatle said.

But Cheatle said she would not resign over the incident, which is set to be investigated by Congress.

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