Visitors to the National September 11 Memorial and Museum will be able to use their iPhone, iPad, or iPod touch to take one of the three official, self-guided themed audio tours, once the museum opens its doors later this month.

According to the museum, each tour features stories from the day of the attacks — September 11, 2001 — as well as the recovery operation at the World Trade Center site. A descriptive tour of the museum itself, which opens on May 21, is also available.

The Robert De Niro-narrated "Witnessing History" tour guides visitors from the perspective of first responders, recovery workers, members of the victims' families, journalists, and other witnesses. A "Discovering History" tour is designed for children from 8 to 11 years old, while a "Building History" tour discusses the architecture, design, and archaeological remnants of the original World Trade Center and the museum.

"Witnessing History" is available in American Sign Language, Mandarin Chinese, French, German, Italian, Japanese, Portuguese, and Spanish, while the other tours are available only in English. The app also supports Apple's VoiceOver functionality for easy navigation by visually impaired visitors.

The audio guide app accompanies the previously-released memorial guide, which helps visitors to the memorial plaza locate the names of friends and loved ones.