A 15-year-old Pennsylvania girl was charged Thursday as an adult in the death of a 14-year-old boy.

Amber Hellesten faces charges of murder, aggravated assault and simple assault for the stabbing of Azim Chaplin, according to the Philadelphia District Attorney's Office.

The charges stem from a Feb. 11 incident in Point Breeze. Police say Hellesten, who reportedly lived down the street from Chaplin, stabbed him once in the chest at about 8 p.m. that night. The blade from the knife pierced the boy's heart and he was transported to Children's Hospital of Philadelphia, where he was admitted in critical condition.

Witnesses at the scene allegedly identified Hellesten to police as the individual who stabbed Chaplin.

The Philadelphia Inquirer, citing unnamed police sources, reported Chaplin was stabbed during an argument with Hellesten and other kids. Tasha Jamerson, a spokeswoman for the district attorney's office, would not confirm that report. She told The Huffington Post the cause of the stabbing is "still under investigation."

Authorities took Hellesten into custody the night of the stabbing and charged her with attempted murder.

"When she was charged with attempted murder, directly after the stabbing, we charged her as an adult because of her actions and because she was 15," said Jamerson.

Pennsylvania law prohibits charging anyone under the age of 15 as an adult in an attempted murder case. At 15, Hellesten met the standard.

Chaplin succumbed to his injuries five days after the stabbing. He was pronounced dead at about 2 a.m. Sunday.

Chaplin's death prompted the district attorney's office to upgrade Hellesten's attempted murder charge.

"Now she's been charged with murder, and in the state of Pennsylvania, if you are charged with murder and you are over the age of 9, generally you are automatically charged as an adult," Jamerson said.

Hellesten is scheduled to appear in court for a preliminary hearing on March 4.