A drunken man spent several hours wandering through the Hart Senate Office Building late Monday night, sparking an internal Capitol Police investigation into the security breach.

The man entered the garage, walked into the main building, then re-entered the garage, and he later was stopped trying to re-enter the main building, Senate Sergeant-at-Arms Terrance Gainer said.

“There was a human error someplace,— Gainer said Wednesday. “He should not have ever been able to get from the garage into the building without being challenged.—

The Capitol Police department is now investigating the incident, but Gainer said he didn’t believe it was a systemic issue.

“I think we interdict a lot, but the officers are human and occasionally there’s an error,— he said. “That’s not a good thing, but in balance they and we do a good job of keeping everyone safe.—

An officer standing guard at one of the Hart building entrances finally stopped the man — described by Gainer as a “misguided drunk— — on his second trip into the building.

The man’s precise journey was not clear until police officials reviewed Hart Building videotape.

Capitol Police spokeswoman Sgt. Kimberly Schneider said the officer “quickly determined that he had no malicious intent— and arrested him for unlawful entry.

But the episode highlights the long-standing problem of securing Congressional buildings that are open to the public.

Members and police officials have long debated how to keep the Capitol complex — assumed to be a top terrorist target — safe yet open. After the Sept. 11, 2001, attacks, police officials briefly considered building a gate around the entire Capitol complex.

Instead, the newly opened Capitol Visitor Center was built to help solve the issue by funneling most visitors through one centralized entrance.

But in some ways, that just increased the need for more officers. Gainer told Senate appropriators last month that the CVC resulted in “more open doors— and drove the need for more officers.

“There’s a lot of ways to save officers here, and we can close a lot of doors to do that,— he said. “But there’s never been the will.—

Right now, the force has 1,800 officers; officials want to beef that up by 89 in fiscal 2010. Meanwhile, some posts are left with one officer, which can mean lengthy lines.

In response to questions about recent long lines outside House office buildings, House Sergeant-at-Arms Bill Livingood told Members that officers sometimes had to be taken from other posts to compensate for the influx of visitors.

“You have to take existing officers and rearrange them,— he said at a hearing of the House Appropriations Subcommittee on the Legislative Branch.

Too few officers at busy doors, he added, is “a serious problem and I empathize with that.—

In recent years, the Capitol Police department has surveyed each of the hundreds of police posts throughout the Capitol campus, determining the specific threat, the volume of traffic and the number of officers needed.

Gainer said his office also keeps up-to-date on any potential vulnerabilities and threats around the world. The Capitol Police, he said, constantly changes its layout on the campus to avoid getting into a recognizable routine. Individual officers also provide a “darn good perspective— on the force’s weaknesses and strengths.

But with 30,000 employees and millions of visitors, mistakes happen, Gainer conceded.

Breaches in security are rare, but over the past few years, several have occurred.

In December, for example, someone broke a window in the first-floor Longworth Building office of Rep. Rush Holt (D-N.J.).

And in 2006, a gun-toting Carlos Greene crashed an SUV on the East Front Plaza and managed to run into the Capitol before being stopped by an employee of the Flag Office.

As for Monday night’s incident, several officers said only one officer stands guard at the opening of the Hart garage at 2nd and C streets Northeast.

But Gainer maintained the incident “appears to be a function of somebody not doing their job.— Investigating the incident, he said, will allow the department to determine how to solve the problem and whether it means more training or corrective action.

“What we want to do is give a good analysis of what happened,— he said.