The son of a New York City police detective murdered in the July mass shooting at a Manhattan office building was born over the weekend, the NYPD said Monday. 

Detective Didarul Islam's widow Jamila gave birth to the couple's new baby boy, Arham, their third son, on Sunday, the department shared on social media. 

Islam was off-duty working security at 345 Park Avenue when a gunman stormed the building in Midtown on July 28. Four people were killed and another was wounded before the shooter turned the gun on himself, police said. 

The 36-year-old was born in Bangladesh and moved to the United States as a child. His family said it was his dream to join the NYPD.

"He always wanted to be a police officer to serve his community, to serve the people around him, and set a better example for his kids," Zamilur Rahman, Islam's cousin, told CBS News New York. 

The couple has two other boys, Ahyan and Azhaan, ages 4 and 7.

"Together, they will carry forward their father's legacy of service and courage — a legacy the NYPD will guard and uphold with the same devotion Didar gave to this city. Jamila, Ahyan, Azhaan, and now, Arham will forever be part of the NYPD family," Police Commissioner Jessica Tisch wrote. 

Security guard Aland Etienne, Blackstone executive Wesley LePatner and Julia Hyman of Rudin Management were also killed in the attack.