But search warrants recently obtained by the Daily News say that Penn told investigators that he heard the triggerman growl: "This is for your wife."
Penn's wife, Jennifer Penn, is also a cop - an administrative lieutenant assigned to South Philadelphia's 17th District.
But the eyebrow-raising words supposedly uttered by the shooter haven't made much of a difference; the shooting remains unsolved, and Jennifer Penn is not a suspect, the department said.
The couple became the subject of an Internal Affairs investigation after being involved in a domestic dispute in December, according to search warrants.
Then, on Monday morning, Jennifer Penn was also attacked outside of the Overbrook house, on Brentwood Road near Haverford Avenue. She told police that a man poured a small container of caustic liquid on her after she climbed inside her car, sources said.
The attacker got away, and Penn wasn't seriously injured. One police source said the container smelled of chlorine.
It's unclear if the two incidents are linked.
The warrants documenting the hunt for the man who shot Robert Penn show that investigators chased a number of potential leads that dried up.
A neighbor was questioned - but never charged - after Jennifer Penn told detectives that the man said to her, "Yo . . . I got guns," and, "Yo, I can't take him [Robert Penn] hitting on you," on March 1, according to the warrants.
Neither Robert nor Jennifer Penn responded to requests for comment.
Little has been said publicly about the case since Robert Penn was wounded.
"As far as the investigation is concerned, we're not getting any kind of information from either the victim or anyone else," Police Commissioner Charles H. Ramsey said.
"The case is pretty much at a standstill," he said. "I don't think we have anybody even close to being charged."
The District Attorney's Office has not received any files related to the case, spokeswoman Tasha Jamerson said.
Robert Penn, 46, told detectives that he and his wife had struggled with domestic problems in the months before he was shot, according to the warrants.
In late December, police were called to the Overbrook house after a domestic dispute, prompting Internal Affairs to get into the case, according to the warrants and police sources.
The outcome of Internal Affairs' involvement with the couple is unclear.
Jennifer Penn stayed with a friend for a while after that incident, but returned to the couple's home on March 1.
The warrants show that she told detectives she came home with the intention of ending her marriage.
She said she went to a neighbor's house. The man - whom the Daily News is not naming because he has not been identified as a suspect - told Penn that she was like a sister to him, and he didn't like how Robert Penn treated her, according to the warrants. "Yo," he said, "I got guns."
Penn told investigators that she assured the neighbor that the intimated threat was unnecessary.
Robert Penn worked a 3 p.m. to 11 p.m. shift in West Philadelphia's 16th District that night. He got home shortly after midnight, and parked his green Ford Expedition in the driveway.
Penn hopped out of his SUV. The gunman approached from behind.
There was no struggle, no attempt to steal the cop's wallet or anything else. Penn heard those five little words - "This is for your wife" - and felt a bullet explode into his right shoulder, according to the warrants.
He later told detectives at the Hospital of the University of Pennsylvania that the force of the blast knocked him to the ground, toward Haverford Avenue. The gunman ran off.
Jennifer Penn told investigators that she was sleeping when someone tried to kill her husband, according to the warrants. She was awakened not by the gunshot echoing across the driveway, but by the steady thrum of a police helicopter that was soon hovering over the neighborhood.
A fellow cop called her and delivered the news.
Penn told investigators that she had three firearms, all of which were at separate locations at the time of the shooting, according to the warrants.
Ramsey said Jennifer Penn was briefly put on desk duty after her husband was shot, but has since returned to active duty.
"There initially was some concern there, but she's not a suspect," he said.
Ramsey said he "heard rumors" about the couple having domestic problems, but "that doesn't translate into her being involved or anything like that."
A cop getting shot outside of his own home is shocking, even in a city as plagued with gun violence as Philadelphia.
But there haven't been any news conferences held or rewards posted to encourage citizens to help detectives nab the man who shot Robert Penn - or the one who attacked Jennifer Penn earlier this week.
Ramsey said there was "no hidden motive" behind the fact that the department has not held news conferences to drum up tips for investigators.
"Penn survived his wounds," he said. "We don't even know if [the shooter] knew he was a policeman."
Rewards, he added, are usually posted by the Fraternal Order of Police Lodge No. 5.
"Most of the time, we're asked to put up a reward," FOP president John McNesby said. "The city points us in a direction if they're stuck on a case. They haven't done that on this case."
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