The probe into the Korean Air “nut rage” incident intensified as the prosecutors appeared to have shifted their focus to determining whether Heather Cho and other employees were involved in destroying related evidence.

The Seoul Seobu District Prosecutors’ Office summoned a handful of employees from the country’s biggest air carrier on Friday as witnesses and implemented a travel ban on them.

Prosecutors were questioning the employees to establish whether they were involved in systematically destroying evidence to minimize the impact of the incident. The authorities were also looking into Cho’s possible involvement in destroying evidence based on the testimonies of employees and their phone records.

The eldest daughter of Korean Air chairman Cho Yang-ho has so far denied that she ordered employees to cover up the case.

Among the Korean Air employees summoned by the prosecutors was the managing vice president of cabin crew operations, known by his surname Yeo. The vice president was booked for playing a leading role in destroying evidence that proved Korean Air tried to cover up the nut incident, such as by deleting internal emails.

Yeo also allegedly attempted to force the deplaned cabin manager and other flight attendants to give false testimony to Ministry of Land, Infrastructure and Transport authorities last week.

The managing vice president reportedly admitted to some of the charges. 

On Tuesday, the Land Ministry said it found Cho to have breached aviation safety laws when she screamed and hurled abuse at a flight attendant and the chief manager Park Chang-jin on Dec. 5, the day she forced a Korean Air commercial flight to return to the gate to drop Park off, thus causing an 11-minute delay.

The following day, Cho was summoned by the prosecution and returned home on Thursday after being questioned for more than 12 hours.

During the investigation, she denied some of the charges, such as ordering the pilot to return the plane to the gate, using violence against two of the crew members and issuing an order to cover up the case.

By Park Han-na (hnpark@heraldcorp.com)