LONG BEACH, Calif. » Police pleaded for the public’s help Monday to find a gunman who shot the parents and uncle of a 3-week-old baby girl who later was found dead in a trash bin near the Mexican border 120 miles from her home.

Investigators have no leads and no motive for the weekend attack, which happened in a densely populated working-class neighborhood of single-family homes, Long Beach Police Chief Robert Luna said Monday afternoon.

The baby’s parents and uncle have spoken with police but have not provided sufficient information —a fact Luna called "frustrating," although he said it was too early to know if the family was withholding clues.

"If you’re talking about a crime that occurred at 6 p.m. on a Saturday in a highly dense, congested residential area, somebody had to have seen or heard something. They may think it’s irrelevant, but if anything seemed out of place … we need to start somewhere," Luna said. "Any small piece of information could be that part of the puzzle that we need to solve this case."

Detectives do not believe the crime was a random act, and the FBI is assisting in the investigation, Luna said.

The Los Angeles County Board of Supervisors is expected to offer a $10,000 reward Tuesday for information leading to an arrest and conviction, he said.

Long Beach detectives have a limited description of the culprit and are looking for a man who is either black or Hispanic. They don’t have a description of a car or a license plate number, so they were unable to issue an Amber Alert over the weekend.

Luna said nothing is being ruled out at this point, including that the gunman has fled to Mexico, or that family members or a drug cartel are involved.

The mother and uncle remained hospitalized in critical condition. The father was released Sunday.

"We don’t have the complete picture yet, but we’re going to get there," Luna said.

A transient found the baby, a 10-pound girl named Eliza Delacruz, dead Sunday in a trash bin behind a strip mall in Imperial Beach, a two-hour drive south of the Long Beach neighborhood where her father, mother and uncle were shot. Mourners erected a makeshift memorial of candles and flowers outside a pizza shop at the mall.

"To take a precious child from home, throw her in a dumpster like a piece of trash, is something only a very heartless person would do," Luna said. "We need to get the coward who committed this violent, senseless act off the street."

The San Diego County medical examiner’s office said it was unknown where or when the infant died. An autopsy had not yet been performed, and no cause of death was released.

Aaron Cruz, who lives next door to the victims’ two-story stucco house, said the baby’s father is a veterinary technician who works six days a week and was thrilled to be a dad.

Cruz, who described himself as the man’s best friend, said the family often hosted large barbecues on their front lawn after moving in a few years ago.

The baby’s parents and uncle lived there with another uncle and the baby’s grandparents, but no one else was there when the shooting happened, Cruz said.

"He had just gotten off work, and this happened to him. It was a normal day for him," he said. "This was out of nowhere."

Cruz recalled a large baby shower with dancing that the family threw in November.

"It was his first and only daughter," Cruz said, as he relit candles that had fizzled out in the breeze. "He was robbed of fatherhood."

A trickle of mourners went by the family’s home Monday. Christmas lights hung on the porch over a string of yellow police tape, and a wreath decorated the door.

Joseph Guerrero stopped with his wife and 4-year-old son to leave a candle and balloon. He didn’t know the victims but was moved by the infant’s death.

"We have three girls and our son, and when we were picking out the balloon, I just squeezed my daughter’s hand thinking about what they were going through," he said.

Associated Press writers Elliot Spagat in San Diego and Amy Taxin in Orange County contributed to this report.