(CNN) Thirteen children of firefighters who died at the World Trade Center on September 11, 2001, and six whose fathers died of 9/11-related illnesses, graduated from the city's fire department during an emotional ceremony.

They were part of a group of 301 probationary firefighters to graduate Tuesday after 18 weeks of intensive training at the New York City Fire Department academy.

"Many of their fathers were killed on September 11, several others died of World Trade Center illnesses, and others lost their lives in the course of their duty, bravely working to save and protect others," said Fire Commissioner Daniel Nigro, who presided over the ceremony.

"Today, these 21 legacies are no longer children dreaming of their careers their fathers loved, and of serving in the world's greatest fire department -- today, they have fully achieved their dreams."

Firefighter Carl Asaro was one of the 343 FDNY members killed after the terror attacks on 9/11.

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