Dozens of people had crammed into the tiny La Tombola bar in Toa Baja, a municipality just west of San Juan, and were waiting for the band to resume playing when the assailants attacked, police Col. Jose Morales said. Among those seriously wounded were a 9-year-old girl and a pregnant woman who lost her 8-month-old fetus, he said.
The justice department plans to file a murder charge for the death of the fetus, said prosecutor Wanda Vazquez, who is investigating the case.
Vazquez said the scope of the killing makes investigators believe the attack may have involved a feud between drug traffickers.
At least two armed men began shooting as they entered, and several people inside returned fire, leaving the bar's newly renovated interior pocked with five different types of bullets, including those from an AK-47 assault rifle, Morales said. The shooting spilled outside before the gunmen fled in a car.
Three people were found dead inside and another three outside, while a seventh died at the hospital, Vazquez said.
As officials carried the uncovered body of one victim, some of the dozens of onlookers began to yell: "That's Jochi! That's Jochi!" while several women began to cry.
Vazquez said few people were willing to cooperate with police, complicating the investigation. "This is a very tight, close-knit community."
Police guarded those who were hospitalized as a preventive measure, although authorities do not believe any suspects are among them, Vazquez said.
Gov. Luis Fortuno visited the shooting victims, and he later called the attack a "senseless act."
"To those responsible for these actions, we are going to follow you, find you and charge you," he said at a news conference.
The killings happened just weeks after federal authorities and local police made several high-profile arrests of suspected drug dealers.
An estimated 30 percent of drugs reaching the U.S. come through the Caribbean, with Puerto Rico a popular transshipment point because drugs do not have to clear customs to reach the mainland.
Authorities say 709 people have been reported killed this year in Puerto Rico, 65 more than during the same period last year. Puerto Rico's police chief estimates that 70 percent of the killings are tied to drug trafficking.
Nearly 4 million people live in the U.S. Caribbean territory.