NEW YORK (Reuters) - Two brothers, age 8 and 9, were charged on Friday in the stabbing death of a 4-year-old boy after they allegedly attacked him outside his New York City apartment and severed his jugular vein with a pen or key, police said.

Emanuel Barima, whose parents emigrated to the United States from Ghana, was fatally stabbed in the neck on Wednesday after he and his 5-year-old sister left their apartment to throw a dirty diaper down a trash chute in the hallway, police said.

The little boy was to have started school on Thursday.

Authorities questioned and charged the two brothers, who live in the same apartment building as the Barimas, after the sister identified them as the attackers and said they had been bullying her throughout the summer, police said.

The 8-year-old boy was charged with criminally negligent homicide and his 9-year-old brother was charged with second degree felony assault.

Both were charged as juveniles. The 8-year-old could face up to 18 months in juvenile detention if convicted. The sentence his brother would face if convicted was not immediately clear.

The case was to go before a Family Court judge in the city's Bronx borough.

Police were investigating whether the boy was stabbed with a set of keys or a pen.

``The weapon went into the victim's neck 1 1/2 inches (3.8 cm) and hit his jugular vein, and he collapsed in his apartment,'' said Bronx Chief of Detectives Joseph Reznick.

Because of the boys' ages it was expected they would be sent to a hospital for psychological examinations, authorities said.