Cara Ellis, 21, who lived nearby and was also killed by Craig Mueller, was hailed as a Good Samaritan, said Ocean County Prosecutor Marlene Lynch Ford.
"Cara was unfortunately caught in the crossfire when she was trying to run to assist Bryan," Ford said of Ellis, who was married and a mother.
"What happened today is a tragedy," said Ellis' brother-in-law Christopher Ellis.
Both victims were hit multiple times and died at the scene.
"At this point, it's just a mystery," Ford said, according to the Associated Press. "We're as shocked at this as anyone else. It makes no sense."
Authorities do not know of any disagreement that might have precipitated the killings. The prosecutor said there were no previous reports of violence at the unit. None of the three victims had criminal records.
Authorities did not know if Ellis knew either of the Mueller brothers.
The first 911 call came in about 10:15 a.m., officials said. Authorities could not determine if the shooter was still in the house, so they released a gas canister in an attempt to force him out if he was, officials said.
Authorities were afraid that the shooter might have run into nearby woods. They started to comb the woods and the surrounding area, authorities said.
But Craig Mueller apparently was still in the townhouse during the entire search.
About four hours or so after the shooting, authorities entered the residence and found Craig Mueller dead in an upstairs bedroom of his unit on Westchester Drive, authorities said.
He had shot himself with the same weapon he used to kill his brother and Ellis, authorities said. Shell casings were found in the bedroom, authorities said.
The community, about 12 miles north of Atlantic City, is normally a relatively quiet area, with many retirees and others employed as fishermen or in other maritime trades.
"This is not what you expect to happen here," said Sheila Milbourne, a Tall Timbers resident, who said she heard about 20 to 30 shots. "It just means that this community is no different than any other community and that violence like this can happen anywhere."
After hearing the gunfire, Milbourne looked out the window and saw Bryan Mueller lying on the sidewalk near her apartment.
"I ran and hid in a closet," she said. "The echoes told me those shots were close. I knew to get away from the window."
Christian Santana, 28, who also lives in the development, was sleeping with his wife and baby in their apartment when they heard the gunfire.
"I looked out the window and saw the body on the sidewalk, and I said, 'Get down, get away from the windows,' " he recalled.
Marni Markoski, 44, who lives on Radio Road across the street from Tall Timbers, said she heard about two dozen shots around the same time, too. She first thought it was someone using a nail gun on a roof.
"In the winter sometimes you hear rifle shots, some people hunting," Markoski said. "But it never sounds like what I heard this morning. That's why it didn't occur to me that it was a gun."
Tall Timbers resident Kelly Sorcen, whose three children are between ages 12 and 16, said she parked nearby and had to sneak into her unit.
"The kids were texting from school, which was very scary. They didn't know what was going on," she said. "Then they were held after school. They took all the kids off the bus who lived in Tall Timbers. We had to go pick them up and show identification."
Schools were locked down until at least the end of the school day, and children were back with their parents by Monday afternoon, according to officials. The closest school was 11/2 miles away, authorities said.
"The public can feel secure that they are in no danger," Ford told reporters Monday.
Ford also said investigators were uncertain whether the AR-15 was legally registered to one of the Muellers.
Families from about 10 units of the two-story townhouse complex were displaced by the investigation and will be housed by the American Red Cross, Ford said.
The Ocean County and New Jersey State Police SWAT teams and the Little Egg Harbor Township Police were among the agencies that responded to the shootings.
The murder-suicide is the second multiple shooting in South Jersey in five days. Thursday night, a Pennsauken man shot his three teenage sons, killing two, and then set his house on fire before he was killed in a confrontation with police.
Contact staff writer Darran Simon at 856-779-3829 or firstname.lastname@example.org.