Chris Inglis, a widely respected intelligence professional, has emerged as Defense Secretary Jim Mattis’s leading choice to head the National Security Agency should Mattis decide to separate its leadership from that of the Pentagon’s cyber forces, according to five people familiar with the matter.

The current head of both organizations, Gen. Paul Nakasone, has urged Mattis to keep the NSA and U.S. Cyber Command under one leader on the grounds that the nine-year-old military organization is not ready to stand on its own, these people said.

In recent weeks, Mattis was close to a decision to separate the leadership arrangement, but Nakasone’s counsel has caused him to reconsider, according to two U.S. officials. The officials spoke on the condition of anonymity to discuss sensitive internal deliberations.

The president would make the final call, but generally accepts the defense secretary’s recommendation.

“No decision has been made,” said Audricia Harris, a Pentagon spokeswoman. “When a decision is made, it will be reflective of the department’s commitment to preempt, defeat and deter malicious cyber activity.”

Inglis declined to comment.

Inglis would be a strong candidate if the NSA moves to civilian leadership for the first time in its 66-year history. He served as deputy NSA director from 2006 to 2014, and over his 28 years at the agency held a series of leadership and operational posts, including special U.S. liaison officer in London and signals intelligence deputy director for analysis and production.

He commands respect from military and civilian personnel across the intelligence and defense communities.

“If they’re going to civilianize the NSA director’s position, he would be my number-one choice,” said Michael Hayden, who led the NSA from 1999 to 2005 as an Air Force general.

Inglis, 63, is an Air Force Academy graduate and a visiting professor in cybersecurity studies at the U.S. Naval Academy in Annapolis, Md. He served nine years with the Air Force and 21 with the Air National Guard, from which he retired in 2006 as a brigadier general.

“He represents a convenient blend of cultures” — of the military and intelligence worlds, said Hayden, who as NSA director, selected Inglis “as one of the two or three people . . . to groom for the most senior leadership.”

The decision to separate the NSA and Cybercom is fraught. There have been efforts over the years to do so, but they always stalled for one reason or another. President Obama was close to making the move toward the end of his tenure but was delayed. In December 2016, with weeks to go before he left office, he issued a statement that “Cybercom has since matured” to the point where it needed to stand on its own.

President Trump awaits a recommendation from Mattis. Nakasone, as The Washington Post previously reported, recommended to Mattis in August that the two organizations remain under one leader for now. He argued that Cybercom still needs intelligence support from the NSA. Others have said that if Cybercom is not headed by the NSA director, the spy agency may not give it the degree of support it needs.

Launched in 2009, Cybercom plays a crucial role in defending the United States against cyberattacks from foreign adversaries, including Russia. It is fully manned with 133 teams and 6,200 people. But in terms of “readiness” or effectiveness, the teams are still considered immature, officials said.

“As the build of the cyber mission force wraps up, we’re quickly shifting gears from force generation to sustainable readiness,” Nakasone said in a statement in May. “We must ensure we have the platforms, capabilities and authorities ready and available” to carry out successful cyber-offensives.

Some former senior intelligence and defense officials oppose separating the “dual-hat” leadership arrangement, including former NSA Director Keith Alexander, former Director of National Intelligence Mike McConnell and former Defense Secretary Robert Gates. This week, former CIA Director David Petraeus, a retired Army general, said during a Washington Post cyber summit that he’d keep the dual-hat arrangement “for the time being.”

But others have argued that to be a full-fledged fighting force, Cybercom needs to have its own leader — not share one with the NSA. They note that the NSA and Cybercom’s offensive missions are fundamentally different. The NSA’s is to spy — to gain and maintain access to computers so it can gather intelligence. Cybercom’s is to disrupt — to thwart an adversary’s warfighting capability.

Under Obama, then-DNI James R. Clapper Jr. and then-Defense Secretary Ashton B. Carter advocated the separation and the appointment of a civilian as NSA director.

If the two organizations split, Nakasone is expected to continue to lead Cybercom, which recently was elevated to full combatant command status on a par with U.S. Central Command, which oversees military operations in the Middle East, and others with purview of the Pentagon’s missions worldwide. He, too, is highly regarded, and no matter what happens, officials have said, the NSA and Cybercom will continue to work closely with one another.