The Secret Service monitored Brenda Allen and her family for months. The surveillance came after an altercation with her neighbor, who worked for former Secret Service Director Mark Sullivan.​ (Theresa Poulson/The Washington Post)

Top Secret Service officials ­ordered members of a special unit responsible for patrolling the White House perimeter to abandon their posts over at least two months in 2011 in order to protect a personal friend of the agency’s director, according to three people familiar with the operation.

The new assignment, known internally as Operation Moonlight, diverted agents to a rural area outside the southern Maryland town of La Plata, nearly an hour’s drive from Washington. Agents were told that then-Director Mark Sullivan was concerned that his assistant was being harassed by her neighbor, the three people said.

Two agents were sent twice a day, in the morning and the evening, to monitor the home of the assistant, Lisa Chopey. The trips began June 30, 2011, and extended through the summer before tapering off in August, according to people familiar with internal shift records.

The agents were members of a surveillance team code-named Prowler, which patrols the outskirts of the White House compound and responds to reported problems. The unit is also tasked with monitoring the southern side of the White House whenever crowds gather to watch the president and first family travel via motorcade or helicopter.

Agents inside the Washington field office were concerned that Operation Moonlight increased security risks to the compound and the president, two people familiar with the discussion said.

The Secret Service and Operation Moonlight

On the first day of the new operation, the two Prowler agents on duty were directed to leave their position on the Ellipse, the public park directly south of the executive mansion, minutes before President Obama departed on his helicopter. The aircraft’s movements on and off the South Lawn are times of heightened security concern.

The agents thought the reassignment was a potentially illegal use of government resources. They were concerned enough about their own liability that they kept records of their involvement and their superiors’ instructions.

Some reported the operation to the inspector general for the Department of Homeland Security, the Secret Service’s parent agency. People familiar with the operation said a Senate committee’s recent finding that the former DHS ­inspector general softened and delayed investigations — particularly those critical of administration officials — renewed frustration that the issue may have not been properly investigated.

White House spokesman Jay Carney said the White House was not aware of the allegations involving the president’s protection and referred questions to the U.S. Secret Service.

Secret Service spokesman Ed Donovan confirmed that Prowler agents were pulled off their White House duty to check on the safety of Chopey in 2011.

But Donovan disputed ­accounts that Operation Moonlight lasted for months, saying agency records indicate that the assignment took place for only a few days over the Fourth of July weekend. Donovan declined to release the records.

He said that the operation was part of the agency’s standard response to potential threats to an employee and that Prowler, as an investigative unit, is not part of the president’s protective detail.

“Because there were no protective assets used during these checks, there was no impact on protective operations,” Donovan said.

Sullivan, 60, who now runs a private security firm, left his Secret Service job last year, 10 months after a controversy over members of the presidential protection team hiring prostitutes ahead of a 2012 Obama trip to Colombia. He said through a spokesman that he did not personally order the 2011 checks on his assistant’s home and that a supervisor in his office authorized the visits. He said that he learned of the checks after they began and that to his knowledge, they were done for just a few days and were “appropriate.”

“The U.S. Secret Service always has taken seriously threats made against employees and responds as appropriate,” Sullivan said in a statement. “In this case, the employee followed protocol in reporting concerns about her safety to a supervisor who took action consistent with the seriousness of the situation. I was informed later of those actions.”

Chopey, 41, who left the Secret Service last year for a position in the office of the DHS secretary, did not respond to requests for comment.

Security concerns have been generally heightened at the White House under Obama, the first black president. He received Secret Service protection unusually early as a candidate in 2007. His wife, Michelle, has expressed worries in the past about his safety, and similar fears have been widespread among many of Obama’s African American supporters.

An alarming reminder of the potential for danger came in November 2011, weeks after Operation Moonlight had ended, when a man who said Obama “needed to be stopped” fired a semiautomatic assault rifle from his car on Constitution Avenue, just a few hundred yards south of the White House. The shots struck the exterior wall and window of the second floor of the family residence on the Truman Balcony, where Obama has been known to wander. The president and first lady were not home at the time; their daughters and Obama’s mother-in-law were.

This past Tuesday, a motorist joined the Obama daughters’ motorcade and drove onto the White House grounds before a barrier could be raised to block him. Prowler agents responded to the incident and interrogated the driver, determining that he was unfamiliar with the city’s streets and made a mistake.

“Prowler is there for a reason, and it shouldn’t be pulled when the president is on the move,” said Dan Emmett, a former Secret Service agent and author of the book “Within Arm’s Length.”

Emmett and other former agents said they were surprised that Prowler or any other units would be deployed for an extended period to check on an employee.

“There is nothing more important than the president’s arrival and departure,” Emmett said. “The president takes far greater priority than the director’s secretary’s well-being.”

The orders for Operation Moonlight came from the top two agents in charge of the Washington field office, David Beach and Jim Donaldson. Both men, who have since been promoted within the Secret Service, declined to comment.

A Secret Service official said the agency’s current director, Julia Pierson, who was Sullivan’s chief of staff at the time, was unaware of the operation.

The episode had its origins in a series of escalating disputes between two neighboring families that shared a gravel road to their homes in La Plata.

On the morning of June 30, 2011, Chopey reported to local sheriff’s deputies that her neighbor, Michael Mulligan, had chased her while she was driving her Ford Expedition and he was on his four-wheel all-terrain vehicle.

Later that afternoon, Donaldson ordered two Prowler agents to leave their posts at the White House, according to two people familiar with the agents’ accounts.

The order came as the agents were in position south of the White House, preparing for Obama’s imminent departure aboard his Marine One helicopter to Joint Base Andrews, where he would board Air Force One for a trip to Philadelphia.

Donaldson was described as telling agents to drive directly to La Plata to perform a “welfare check” on Chopey’s home.

Chopey later sought and received a restraining order against Mulligan. Mulligan disputed Chopey’s account, saying that Chopey tried to hurt him with her SUV when he had been trying to catch up to her to discuss tensions between the families. He later pled guilty to misdemeanor assault ­under a special plea agreement in which he insisted he was innocent of the crime but acknowledged that the evidence against him could lead to a conviction.

The Prowler agents’ trips to La Plata continued through the summer months, with pairs of agents sent as often as twice daily to check on Chopey’s home and on Mulligan’s whereabouts.

The agents recorded their shifts by writing their initials in a Washington field office log stored in a manila folder stamped “Moonlight.”

The presence of imposing-looking unmarked vehicles, including a black SUV or an American-made sedan, came as a shock to Mulligan and his then-girlfriend, Brenda Allen. They said they did not know who owned the cars that parked behind the tree line near a backyard shed.

“There was all these cars down there for months,” Mulligan said. “They parked everywhere. It actually scared us. I wasn’t sure if it was police or what.”

Mulligan and Allen said they approached one of the cars one day to ask the occupants why they were parked in front of the house. But they said the car sped off.

The couple said they barely went outside their house when the cars were there. Eventually, the mysterious surveillance and the tensions with the Chopey family led them to move, they said.

Over time, the agents’ trips to La Plata grew less frequent, according to people familiar with the work, and by early fall, Operation Moonlight was over.

Mulligan and Allen said they did not know they had been under Secret Service surveillance until informed last week by The Washington Post. A spokesman for the Charles County Sheriff’s Office said the office was not aware of the Secret Service making security visits to Chopey’s home.

“I don’t think the Secret Service should have been down to my house, watching me all the time,” Allen said. “What threat do we pose? Why so much attention for this woman?”

Alice Crites contributed to this report.