Lopez's actions last August certainly generated an investigation.
After his son rear-ended a car, he came to the 8th District, in the Northeast, where Lopez worked, and reported the fender-bender. Lopez then took his son in his patrol car and found the other car involved parked at a Lukoil store at Comly Road and Roosevelt Boulevard.
A surveillance video showed Lopez entering the store with his son in tow and quickly pulling his gun. He jammed the weapon into the face of Agnes Lawless, 20, hard enough to leave a bruise. After a chaotic struggle, he called for backup and arrested her on charges of assaulting an officer.
The case against Lawless fell apart after the store's surveillance video was released. Lopez is now on desk duty awaiting a hearing on his conduct. Vanore said Philadelphia has a clear policy on how off-duty officers should behave when they encounter a personal situation that could merit police intervention.
Unless immediate action is necessary, Vanore said, an off-duty officer "should call 9-1-1 and act as a witness."
Police authorities have long recognized the risk of officers' becoming involved in situations in which they have a personal stake, both on and off duty.
"Where a family member is involved, or a good friend, even a friend of a friend, you really should turn the case over to another officer," said Tod Burke, a former police officer and professor of criminal justice at Radford University, in Virginia.
Within the last year, two incidents in Philadelphia involved officers' intervening in personal situations, with both leading to violence.
In November, Sgt. Chauncey Ellison went looking for some young men in West Oak Lane who had assaulted his 14-year-old son and robbed him of a pizza.
Although he was off duty, Ellison brought his service weapon and allegedly shot Lawrence Allen, 20, who was unarmed. Allen died three months later.
Ellison is now on desk duty. The District Attorney's Office has not decided whether to charge him in the shooting.
Officer John Safarowicz was dismissed from the department after an incident last September in which he allegedly used his badge to gain entrance to a Roxborough home and assault a man who lived there.
According to court documents, Safarowicz was looking for two men who had assaulted his brother-in-law.
In a number of cases, police officers also have been accused of using their authority to intervene in child-custody disputes, on behalf of themselves or a relative.
Philadelphia's policy on off-duty conduct dates to 1998, and it followed several lawsuits involving officers accused of abusing their authority in personal disputes.
A key case involved two officers - Francis Cheney and Paul Viola - accused of beating James Maryanski and Brian Kelly after a taproom altercation and car chase in the Northeast in 1992. According to trial testimony, Maryanski had a bat, but dropped it after the two men identified themselves as police officers.
Maryanski's attorney, Alan Yatvin, introduced a trial exhibit listing 26 cases in which officers had been found abusing their authority in personal disputes.
Both officers were fired and prosecuted for assault.
But a federal jury found that the city's failure to properly train, investigate or discipline officers for off-duty misconduct had contributed to the incident.
Partly as a result of the case, the department developed an off-duty-conduct policy, which was issued in a memorandum by then-Commissioner Richard Neal in 1998.
Current Commissioner Charles Ramsey declined to release a copy of the memo to the Daily News, and declined a request for an interview.
Yatvin said that the department would have spoken more forcefully on the issue if it had issued a formal police directive or had changed the disciplinary code, steps that carry more weight than a commissioner's memo.
Yatvin also thinks the department should specifically address how on-duty officers are to handle situations in which they have a personal interest.
"There is no immediate appeal from what a police officer does while he's on the street," Yatvin said. "So it's important that the department make sure officers know what their conduct is supposed to be."
In cases like that of Officer Lopez, who believed that his son had been wronged, Yatvin said the department should have a clear message:
"The department needs to say, 'You don't take things into your own hands. You call someone whose son was not involved. You don't go into a gas station, angry and cranked up about an incident.' "