(CNN) As Tamir Rice's 14-year-old sister rushed to her brother's side upon learning he'd been shot, police officers "tackled" her, handcuffed her and placed her in a squad car with the Cleveland officer who shot Tamir, her mother and a Rice family attorney told reporters Monday.

The mother, Samaria Rice, was threatened with arrest herself as she "went charging and yelling at police" because they wouldn't let her run to her son's aid, she said.

Speaking at a Baptist church in Cleveland, Rice recalled how a seemingly normal November 22 morphed into tragedy as two Cleveland police officers pulled up to her son outside a recreation center across the street from her home. Within two seconds of exiting the police car , Officer Timothy Loehmann gunned down Tamir, 12. The boy died the next day.

Tamir was playing with a pellet gun, and a witness who saw "a guy with a pistol" told 911 twice that it was "probably" fake but that Tamir was scaring people. It doesn't appear the 911 dispatcher relayed the information to Officers Loehmann, 26, and Frank Garmback, 46.

Police have said that Loehmann, who has been criticized for his policing in the past, opened fire after Tamir reached for the gun in his waistband and that an orange tip indicating the gun was a toy had been removed. Rice said she didn't allow her son to play with toy guns, explaining that one of his friends had given it to him.

A harrowing knock on the door

The boy's mother recalled Monday how she got the news that the youngest of her four children had been shot.

"Two little boys came and knocked on my door and said, 'Police officers just shot your son twice in the stomach,' " she said.

At first, she didn't believe them but quickly realized they were telling the truth.

"I really thought they was playing, like joking around, but I saw the seriousness in their face, and it scared me," she said.

She ran to the scene, admittedly frantic, and arrived at the same time as an ambulance. Officers wouldn't let her check on her son, she said, "and then I saw my daughter in the back of a police car, the same one the shooter got out of." Family attorney Walter Madison said police placed Tamir's sister in the car with Loehmann.

Samaria Rice said she calmed down and asked police to release her daughter. They told her no, she said. Not only would they not release her daughter, but later, she said, they made her choose: Stay with her daughter or accompany her son to a hospital.

She chose the latter but was told she couldn't ride in the back of the ambulance with her son, so she rode in the front seat on the way to the hospital, she said.

"The treatment of the family is unacceptable," said Councilman Jeffrey Johnson, who appeared alongside the family at the news conference. "It just shows the lack of training when we shackle a grieving sister, threaten a grieving mother and not even take care of a child lying on the ground."

Cleveland police declined to discuss the family's allegations. Detective Jennifer Ciaccia told CNN, "We're really not commenting further at this point."

In a lawsuit filed last week against the city and the two officers, the family says Loehmann and Garmback "refused to provide any medical attention to Tamir for at least four minutes as he lay on the ground alive and bleeding."

Cleveland police Chief Calvin Williams has previously said that four minutes after Tamir was shot, a detective and FBI agent arrived and the FBI agent administered first aid. Paramedics arrived three minutes later, the chief said.

Attorney: Brown, Garner cases aren't templates

Attorney Benjamin Crump said the Rice family is "very distrustful" of the justice system, and in light of the grand jury rulings in the Michael Brown and Eric Garner cases, is demanding a transparent investigation. In contrast to the Brown and Garner cases, the family also wants the officers -- who are on paid leave -- charged before a grand jury hears the case, said Crump, who also represents the Brown family.

"There is nothing written anywhere in the law that says police officers are to be treated differently from any other citizen," Crump said. "We cannot have children playing cops and robbers on a playground and police officers coming and claiming their lives."

Tamir was by himself in a gazebo when Garmback and Loehmann pulled onto the grass alongside the gazebo and got out of their car. From the dispatcher's failure to relay the report that the gun was probably fake to the haste with which Loehmann shot the sixth-grader, Crump said "several things were done inappropriately," which is sufficient probable cause to charge the policemen.

The family attorneys also called for the ouster of Safety Director Michael McGrath and Martin Flask, executive assistant to the mayor -- a call echoing one by Councilman Johnson, who asked for their resignations in a Cleveland newspaper last week after a Justice Department report that said Cleveland police had a pattern of excessive force

The family's primary objective, Crump said, is to "hold the killer of their child accountable."

"Tamir was a bright child. He had a promising future," his mother said, explaining that he was a talented artist, drummer and athlete.

Asked what would represent justice in her eyes, Samaria Rice replied, "I'm actually looking for a conviction."