US military, intelligence and law enforcement officials were briefed late last month on a threat against the Pentagon's most senior leaders while they are on American soil, not just traveling overseas, five senior US officials with direct knowledge of the matter told NBC News on Thursday.

Some officials said the briefings suggested the threat, which remains active, may be potential retaliation for the US military's assassination of Iranian General Qassem Soleimani in January, although the information provided did not draw a definitive link.

Iran retaliated for the killing of Soleimani by launching a barrage of missiles on two Iraqi military bases hosting American troops. 34 service members were diagnosed with traumatic brain injury as a result of the attack.

However, there have been reports that Iran seeks to carry out another act of revenge, including a plot to assassinate the US ambassador to South Africa. Iran denied that report.

The recent briefings, according to NBC News, have included information that suggests the targets of the threat are US military leaders who were involved in the decision and operation to assassinate Soleimani, officials said.

The briefings have also included information about a list, compiled by adversaries, of the names of military leaders who are to be targeted, according to two senior US officials.

The briefings of FBI, CIA and military officials on the threat took place after an incident on the evening of September 22 involving a senior leader at the Defense Department, officials said.

The Defense Department leader left the Pentagon that evening in a government-owned black SUV driven by a member of his security detail, when an unknown vehicle immediately began to follow them, officials said. The driver, identified as an Iranian national, was in a vehicle with Virginia license plates and trailed closely behind the official SUV for five to seven miles, at times driving aggressively, according to officials who described a report on the incident that was created by the Pentagon.

The Pentagon and the FBI disagree about whether it was a serious attempt to target a senior Defense Department leader, officials said, with the Pentagon raising more concern.

The FBI investigated the incident and determined it was not part of any larger threat to senior military leaders or connected directly back to Iran, officials said.

One senior administration official said the incident was "concerning."

The security detail for the Defense Department leader who was followed last month when leaving the Pentagon eventually lost the trailing vehicle and changed routes, officials said. The Pentagon subsequently issued a Be on the Lookout alert, called a BOLO, and shared the information with federal law enforcement, officials said.

They said the alert included photos of the driver and the vehicle, as well as the vehicle's license plate. Briefings that officials received on the incident included information about the driver's Facebook and Instagram accounts, showing that he has friends in Iran and Afghanistan, two officials said.

Officials would not say whether the man was questioned or taken into custody.

“The Department of Defense takes seriously the safety of all of our personnel,” said Pentagon spokesman Jonathan Hoffman. “We will not discuss intelligence regarding potential threats to senior leaders nor the range of force protection measures we have in place to address these threats.”

Hoffman also said that for international travel by Defense Department officials, "security measures [are decided] on a case-by-case basis in coordination with appropriate US and host nation law enforcement officials to protect our hosts, traveling officials, their support staff and accompanying media. We are constantly evaluating the threat environment.”

The CIA and FBI declined to comment.