In each instance, the unarmed bandit handed the teller a demand note, received an undisclosed sum of money and ran, authorities said. No one has been injured.
"We don't typically see serial bank robbers today in Philadelphia getting the number of robberies up too high," said FBI special agent J.J. Klaver. "That's because we tend to catch them."
Unlike most robbers, Klaver said, the man has targeted locations that are spread out geographically. His first, Feb. 2, was a Citizens Bank inside an Acme Market on City Avenue. Surveillance footage shows him wearing a puffy down coat with a green baseball cap.
Five days later, he struck at a Citizens Bank in Aldan, Delaware County. He then pulled off three heists and one failed attempt over the next five days, including a bank in Delaware, authorities said.
On Valentine's Day, he robbed a Sovereign Bank on the Main Line a half-hour after a failed attempt at a TD Bank in Wissahickon.
On Sunday afternoon, the elusive bandit hit the same TD Bank in Wissahickon that he'd left empty-handed three days earlier.
Despite circulating high-quality surveillance images - many of which show the man's face clearly with black-rimmed eyeglasses and a goatee - authorities have yet to identify him. They said it's just a matter of time.
"He's making no attempts to hide his face. That actually is not that unusual," Klaver said. "We don't see too many robbers who wear masks or make outrageous attempts to cover their faces.
"We do get hats and scarves and sunglasses . . . " Klaver said. "We certainly don't want him to start [wearing a disguise]."
On Wednesday, after talking with other area law-enforcement agencies, federal investigators discovered at least four other incidents that they believe involved the goateed bandit.
"It's not definitive that he's responsible for every one, but it's certainly looking fairly strong that he's a suspect in all these, so it helps to consolidate everyone's efforts," Klaver said.
Although investigators said that they have not received a description of any vehicle leaving the scenes of the bank jobs, they believe the robber has a car, based on the widespread nature of the robberies. He also has not been seen with a weapon, nor has he claimed to have one, Klaver said. Still, investigators describe him as armed and dangerous.
"We're always worried about violence," Klaver said. "We've seen that in the past. We don't want this to escalate and we don't want someone to get hurt."
Serial bank robbers in Philadelphia are nothing new. In 2010, Hiram Joseph Adams, the so-called "Mummy Bandit," pulled four heists and attempted a fifth before federal authorities collared him. He pleaded guilty.
In 2011, police arrested a trio wanted for a string of robberies at cellphone, food, shoe and convenience stores across the city. One of the men pleaded guilty, but charges were dropped against the two others.
Experts said serial robbers generally keep up the ruse until they get caught.
"They can't continue to do it, because at some point they're going to make a mistake or eventually the bank is able to [turn] on the holdup alarm safely before he exits the bank," said Richard F. Cross, a former security director for the Bank of New York.
There is a common perception that bank robberies rise in bad economic times, but the numbers don't necessarily bear that out, Cross said.
In 2011, Philadelphia had 72 bank robberies, the FBI said. Last year, there were 74. In the first month and a half of this year there have been 15, slightly above the pace over the same duration in the preceding two years.
Authorities declined to say how much money the serial robber has stolen. Based on the images, however, they believe someone can identify him and provide investigators with the tip they need to nab him.
"We're looking for some tips on who this person might be, even if someone is not sure," Klaver said. "If they think this person may look like someone they see regularly, let us know and we'll follow up and try to make the identification."
The robber is described as a black man in his late 20s to early 30s, about 5 feet 10, with a medium build, medium to dark complexion, a goatee and glasses.
Tipsters, who can remain anonymous, should call the FBI at 215-418-4000, or Philadelphia Police at 215-686-TIPS. A reward may result from information leading to an arrest.
On Twitter: @ChroniclesofSol