By Kristen Holmes, Kevin Liptak, Holmes Lybrand, Alayna Treene, Betsy Klein, CNN

(CNN) — President Donald Trump’s decision to depart Turkey on an older plane while sending his new, Qatari-gifted jet ahead to England was driven at least in part by security concerns around the escalating conflict in Iran, according to four US officials.

Two of the officials said security personnel felt more comfortable with the president aboard the older vessel — which was built from scratch with the commander-in-chief’s safety in mind — rather than the plane that had recently been retrofitted after it was donated by Qatar.

While sources said there was no specific, new threat to Trump’s life, the president had mentioned the possibility of an assassination attempt from Iran during the NATO summit, following fresh US strikes on the country that shares a border with Turkey.

“I’m number one on the list for killing,” he said.

The New York Times previously reported on the security concerns. CNN has reached out to the White House for comment.

The specific reason why the older plane was considered a safer option than the newer one was not immediately clear. Two sources familiar with the matter said the newer plane did not possess the same capabilities to ensure its safety in international settings, though a third said it was equipped with many of the same features as the older plane.

“Threat assessments change all the time,” one source said.

Before Trump left the NATO summit in Turkey, officials sent the Qatari-gifted jet ahead to England’s Mildenhall Air Force Base. Trump said in a post on social media that the change in planes was simply to give US service members stationed at the base “a chance to tour the Aircraft.”

“Everybody is so excited, and we thought that they should be the first,” he wrote.

Trump then switched planes at a secure US airbase in the UK and offered mixed messages on the reason for the change of plans.

“There wasn’t a security concern, except we sent it a little early, same line going back. We sent it a little bit early, so that we could let them see,” he said.

But when asked why reporters aboard the plane were asked to lower their window shades on the ascent out of Ankara, Trump allowed that security concerns related to Iran could be a factor. He said Air Force One could be on a “dangerous plane because of the sleaze bags we have to deal with.”

“These are sick people, so I could see something like that,” he said, adding that he was unaware about the directive to press members to keep the shades down.

The new jet, a gift from Qatar estimated to be worth about $400 million, has long generated legal, ethical and national security questions, though the Air Force has said it “is safe, secure and equipped with the most advanced technologies necessary to meet the requirements of the presidential mission.”

One official, who declined to comment on the specific highly classified systems typically onboard Air Force One, noted the 747 gifted by Qatar was largely seen by the military and Secret Service as having been “rushed” into service.

By contrast, the official noted two new jets acquired as part of a deal Trump renegotiated during his first term have faced serious delays and aren’t expected to be ready until at least 2028. That’s partially due to the host of classified communication and defensive countermeasures that must be affixed, comprehensive design requirements by the military and the time required to train pilots on the unique aircraft.

“This was always considered a ‘bridge plane’ that the president would use until the other planes were fully loaded,” a different source said of the Qatari plane, comparing it to the pair of new Boeing jets that have been in the process of being modified for years. The Qatari jet, by comparison, took only roughly a year and a half to modify, which concerned some security officials at the time.

That source noted that in private conversations long before this trip, top White House officials had discussed using the older plane in potentially high-risk international travel, as opposed to the Qatari jet, if needed.

“The new plane is perfectly safe for the president to fly on,” the source said, but added that there are extra precautions needed when flying to high-risk areas internationally.

The older plane that took Trump out of Turkey, a heavily-modified Boeing 747-200, has highly specialized capabilities, including mid-air refueling.