The Department of Homeland Security (DHS) has long suffered from the mystery of morale morass.

Like all mysteries, this one has a cast of characters. They aren’t actors, but many of them are acting. That makes this such a morass.

The department’s Web site shows 40 percent of positions on the DHS’s leadership list are filled by “acting” officials or are vacant — including the top four slots.

“This means nearly half of the top positions at the third-largest department in the U.S. government are not filled,” House Homeland Security Committee Chairman Michael McCaul (R-Texas) told a hearing. “[P]ut more simply, who is in charge?”

“While DHS has thousands of dedicated career employees, it is suffering from a void of leadership,” he added.

That should begin to change now that the Senate confirmed Jeh Johnson as DHS secretary Monday. The deputy secretary job probably will be filled soon, too. Johnson knows the mess he’s getting into. At his confirmation hearing, he said the DHS leadership vacancies are “of alarming proportions.”

“Acting” officials, some talented, can get the basic job done. But their temporary status puts them at an inherent disadvantage. If they are caretakers who don’t want or don’t expect to get the job they are temporarily filling, they are unlikely to provide the imaginative leadership agencies need. If they want the gig, they have to be careful about appearing too presumptuous, again leading to uninspired leadership. Either way, employee morale can suffer.

“And while Acting Secretary (Rand) Beers and other acting executives have worked diligently in recent months,” Tom Ridge, the first DHS secretary told the committee, “you simply cannot build nor can you sustain a mission-focused culture with a high number vacancies and leaders in non-permanent status.”

DHS lacks a “sense of urgency,” he said, and that “undermines mission and morale.”

The complex history of the department often is cited as a reason for its poor showing. When DHS, now with 240,000 employees, was organized in 2003 “we were working to create a unique and unified department culture out of 22 agencies . . . a daunting challenge,” Ridge said. “This has remained a challenge in the department’s first decade.”

Curiously, the acting DHS leaders were not invited to testify at the hearing. In a statement to The Washington Post, DHS said: “The Department’s employees are our greatest resource and we are committed to providing them with the resources and support they need to carry out our mission to protect the homeland. . . . We are focused on continuing to improve employee engagement through enhanced communication and training, employee recognition and strengthening the skills of employees at every level.”

Colleen M. Kelley, president of the National Treasury Employees Union, agrees DHS morale has been undermined, but she thinks leadership vacancies are not the main reason. In testimony to the committee, she said “a significant cause of low morale” at DHS’s Customs and Border Protection is “ongoing staffing shortages.”

While “management deficiencies . . . do contribute to low morale among federal workers,” Kelley said she “believes that government-wide morale problems can be traced directly to the three-year pay freeze, the continuing impact of sequestration and the furloughs it spawned, and the 16-day government shutdown.”

But those things hit feds across the board. They don’t explain the department’s repeated residency near or at the bottom in federal employee surveys. It ranked at the bottom of 19 large agencies in the Partnership for Public Service’s 2012 Best Places to Work in the Federal Government.

In the Office of Personnel Management Federal Employee Viewpoint Survey, DHS was 36 of 37 agencies in the leadership/knowledge management and job satisfaction categories, according to the Government Accountability Office (GAO).

Last year, GAO recommended that DHS “take action to better determine the root cause of low employee morale.” In a report to the committee, GAO said: “DHS has actions underway . . . but DHS has not fully implemented them.”

One such action, “directing component human capital officials to reevaluate their action plans to ensure that metrics of success are clear and measurable,” itself sounds like a bureaucratic morale buster.

There is at least one good sign — the vacancy rate for DHS senior executives has declined significantly since 2006, when it was 25 percent. It had dropped to 10 percent by the end of fiscal year 2011.

Yet, why DHS continues to be a bottom-feeder remains a mystery.

Said GAO: “We concluded that the gap between DHS and government-wide scores may be explained by factors unique to DHS, such as management practices and the nature of the agency’s work, or by differences among employees we could not analyze.”

Twitter: @JoeDavidsonWP