Former NTSB investigator Dr. Alan Diehl tells Fox News the search for answers in the Philadelphia plane crash may prove difficult due to “fragmentation.”

“The cockpit voice recorder will be critical,” Diehl said.

Officials are still searching for the device as of Sunday.

“[The investigation] will be very difficult if that voice recorder is not intact, or they can’t find it,” he added, noting it “should have” survived the impact but that the violent nature of the crash could make the search difficult.

Mexican President Claudia Sheinbaum later confirmed that all six people onboard the flight, operated by Jet Rescue Air Ambulance, were Mexican citizens. The seventh person killed was on the ground.

Philadelphia residents are shaken up after the fatal crash of a Learjet 55 on Friday evening.

A resident named Lena told Fox29 the sound “was terrible.”

“In my life I never heard this kind of sound,” Lena told the outlet. “My ears are still buzzing. I was driving five minutes before. I could’ve been here if I had been driving.”

Another resident, Angelique, said the crash happened on her niece’s daily route to and from work.

“Just imagine, she got off late yesterday,” Angelique told Fox29. “If she hadn’t got off late yesterday, she could’ve been in this.”

Authorities have identified all six people who were on board the Learjet 55 that crashed on Friday evening in Philadelphia less than a minute after taking off.

Captain Alan Alejandro Montoya Perales and Copilot Josue de Jesus Juarez Juarez were flying the aircraft, which was transporting the young patient Valentina Guzman Murillo and her mother Lizeth Murillo Ozuna to Tijuana.

Dr. Raul Meza Arredondo and paramedic Rodrigo Lopez Padilla were also on board to care for Valentina, who had just completed life-saving treatment at Shriners Children’s Hospital.

“It’s extremely hard and extremely difficult, those that were involved directly in her care were very aware that she was going to be traveling home and there had actually been a sendoff for her today,” hospital spokesperson Mel Bower said in a statement.

Investigators are still searching for the black box from the Learjet 55 that crashed on Friday evening less than a minute after taking off.

“It could be intact,” National Transportation Safety Board Chair Jennifer Homendy said. “But likely it is damaged. It may be fragmented.”

According to Fox29, those who need help in the wake of the deadly crash can contact the Red Cross’s free, 24/7, multilingual Disaster Distress Helpline: 1-800-985-5990

Mental health services are also available through Philadelphia’s Department of Behavioral Health and Intellectual Disability Services. Call (215) 685-6440 or visit DBHIDS.org.

The Disaster Distress (DDH) Helpline is also available: 1-800-985-5990.

Residents should not touch any debris they find, Fox 29 reports. To report debris, contact the National Transportation Safety Board at: witness@ntsb.gov.

Fifteen of the 19 people injured in the crash are being treated at Philadelphia’s Jefferson Health, Fox 29 reports.

Twelve of them have been discharged, two were admitted for further care and one has been transferred to another hospital, a spokesperson for Jefferson Health said Sunday.