She walked home from school with her dad Joseph and waited desperately to hear from her mom Cynthia, who worked across the street from the World Trade Center.
Gaga admits she feared the worst for her mom, who was out of contact for 10 hours when phone lines went down.
The pop star tells radio host Toby Knapp, "The whole city was covered in ashes."
She also shared the mixed emotions she felt after learning about the death of terrorist mastermind Osama bin Laden at the weekend (30Apr11-01May11).
Gaga explains, "It's a difficult thing - nobody wants to celebrate anyone's death. But I know it was something that meant a lot to the people of New York, especially those who lost (the) lives (of loved ones) that day."
Reports that bin Laden had been gunned down in Pakistan emerged on Sunday (01May11) and Gaga insists it was "an historical moment in the fight against hatred".
The singer's interview is due to air on Friday (06May11) on Washington, D.C.'s Hot 99.5.