With no leads and no motive, investigators have pleaded for the public's help to find a gunman who shot and wounded 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, police say.

Police Chief Robert Luna said the lack of direction in the investigation and the dearth of information from the injured victims was frustrating.

"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," Luna said at a Monday news conference.

The baby's parents and uncle have spoken with police but it was too early to know if the family was withholding clues, Luna said.

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 body of the baby, a 10-pound girl named Eliza Delacruz, 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.

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."

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Associated Press writer Elliot Spagat in San Diego contributed to this report.

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Follow Gillian Flaccus at http://www.twitter.com/gflaccus.