First lady Jill Biden recalled over the weekend how she was overwhelmed with fear that her sister might be on one of the four hijacked airplanes on Sept. 11, 2001.

Biden spoke of her sister, Bonny Jacobs, who was a flight attendant for United Airlines, and her emotions when the first plane crashed.

"I called Bonny to see where she was because I was scared to death ... I didn't know where she was, whether she was flying, not flying, where she was," Biden recalled in an interview Saturday with the Associated Press. "And then I found out she was home."

Biden, who at the time of the attacks was teaching at Delaware Technical Community College, said she went straight to Bonny's Pennsylvania home after learning she was safe.

On Sunday, Biden and Jacobs also spoke of their scare together during a speech at the United Flight 93 National Memorial in Shanksville, Pennsylvania. Biden said that although Jacobs was not affected, the crashes still had a profound impact on her sister.

"It's a job that she's loved for many years, and I knew that the weight of this tragedy would be heavier for her. When I got to her house, I realized that I was right: She hadn't just lost colleagues — she had lost friends," Biden said in the speech.

Biden also highlighted how she believes the "humanity" of the people who responded on 9/11 overpowered the "inhumanity" of the attacks.

Jacobs spoke of how Biden has offered her strong support during a tough time.

"She was there for me at the time that it happened and she actually is always there for me. She is my rock. Everybody should have a rock in their life, and she is mine," Jacobs said of Biden.