During an afternoon news conference at Police Headquarters, District Attorney Seth Williams said the two could be sentenced to life in prison.
"What they did was indefensible and unforgivable," said Homicide Capt. James Clark, who spoke at the news conference along with Police Commissioner Charles H. Ramsey and other officials.
Ramsey called it a "terrible, terrible event that took the lives of three innocent children."
Rosa and Crawford were taken into custody Sunday night, concluding a citywide manhunt launched Friday morning after a carjacked Toyota 4Runner lurched out of control and into a family selling fruit at Allegheny and Germantown Avenues.
The crash killed 15-year-old Keiearra Williams, 10-year-old Thomas Joseph Reed, and 7-year-old Terrence Williams. Their mother, Keisha Williams, was critically injured and remained Monday night at Temple University Hospital.
Williams announced that NBA great Charles Barkley had offered to pay for the children's funerals.
Another woman who had been selling fruit with Williams and her family, Thelma Brown, 69, suffered a broken ankle and a bruised sternum, and remained Monday at Hahnemann University Hospital.
Also injured was the 45-year-old real estate agent who was carjacked, and sexually assaulted by the men after they forced their way into her car.
According to a police report, the two took $3,000 from the woman, who suffered broken ribs, cuts, and bruises in the subsequent crash. She was also taken to Temple, where she remained Monday in stable condition.
Police and city officials on Monday thanked Detective Ed Tolliver, the assigned detective, and Sgt. Charles Coan for working "nonstop" to solve the case.
"No one wanted to go home," Clark said. Veteran detectives, some assigned to the unit for 15 years, said it was one of the saddest cases they had ever seen.
Clark called the arrests a "great collaboration between the community and police," and detailed the many tips that came in soon after the crash.
He thanked the public for tips and video footage that poured in as news of the crash spread Friday. He said several witnesses gave police detailed descriptions of Rosa and Crawford, and identified them once they were in custody.
Ramsey said one of the first witnesses on the scene - a man who helped pull the real estate agent out of the battered 4Runner - had found a cellphone inside the car and initially mistaken it for his own, an identical model.
Once home, Ramsey said, the man realized that the phone was not his and called 911. Police traced the phone to Rosa and took him to the Homicide Unit for questioning Saturday afternoon.
Law enforcement sources said late Monday that both Crawford and Rosa were talking to police.
Police said Crawford and Rosa walked up to the real estate agent after she wrapped up a meeting with a client on North Sixth Street, about a mile from the crash site. They implied they were carrying a weapon, police said, then forced her into the car and drove north at high speeds.
At no time was the 4Runner being pursued by police, Clark said.
Crawford was driving, police said, when the car went out of control, blew a tire, and careened into Williams and her children, who had been selling fruit to raise money for a neighborhood playground, police said.
Clark said Rosa and Crawford had known each other for only a week, and he did not say how they met. Rosa has never been arrested before. Crawford has a long criminal history, with 10 prior arrests for robbery, theft, and forgery.
Crawford was paroled a year ago after serving six years of a six- to 15-year sentence in connection with a 2007 robbery spree. Records reviewed by The Inquirer show that at the time of his release, state Department of Corrections and parole board testing had determined that he had a "high risk of violent re-offending."
Rosa denied any involvement with the crash, Ramsey said, and told police he had misplaced his phone. Without enough evidence at the time to hold him, Ramsey said, he was released.
Ramsey said Rosa and Crawford then went to Rosa's mother's home and asked to stay there for a few days. Rosa's mother refused to allow Crawford to stay, Ramsey said. And with Crawford gone, Ramsey said, Rosa broke down and told his mother of his involvement in the carjacking.
Rosa surrendered to police Sunday, accompanied by his mother and pastor.
Rosa's lawyer, Christopher Warren, said pictures of the Williams children published in a newspaper had prompted his client to tell the truth. He said his client was a high school graduate and had been planning to join the Marines in the fall.
"He cannot shake from his head the mental image of that girl flying across the hood of the SUV," Warren said.
Warren said his client was not driving the car, and denied that he assaulted the real estate agent.
"He did decide to get in the car," he said. Warren added that his client is "not even thinking about defenses right now."
"Personally speaking and on behalf of my client, condolences extended to the Williams family is really the only thing we're thinking about right now," Warren said.
On his Facebook profile, Rosa said he had studied health information management at Jules E. Mastbaum Area Vocational Technical School. Under "employment," he had "Former Contract Killer" at "swaqqd'up society."
Crawford was apprehended by U.S. marshals and homicide detectives Sunday night behind an abandoned house a half-mile from the carjacking site. A law enforcement source said he was hiding in a shed and was taken without incident.
His extensive criminal history began in earnest in 2007, when at the age of 16 he was sentenced to six to 15 years for a robbery spree that year.
In a 10-day period, court records show, Crawford committed three robberies four blocks apart. The records show he accrued eight serious misconduct offenses while in prison, most during his first months of incarceration.
He was paroled on April 7, 2013 - just four days after he reached his minimum sentence - and released to a North Philadelphia halfway house.
According to records released Monday by the state Board of Probation and Parole, Crawford was granted parole after demonstrating "motivation for success" and "positive institutional behavior."
He also got a positive recommendation from the state Department of Corrections.
Crawford, however, violated the terms of his parole, and in August 2013 was transferred to Coleman Hall, a parole violators' center in Juniata Park. His parole was revoked because he had "changed his residence without permission" and was ruled "not amenable to parole conditions," according to the state Board of Probation and Parole. He was released from Coleman in February of this year and settled in Logan.
One law enforcement source said that Crawford was visited frequently at his Logan home by parole officers in recent months and was last seen by a parole officer two weeks ago.
Inquirer staff writer Joseph A. Slobodzian contributed to this article.