(NewsNation) — Transportation Secretary Pete Buttigieg announced an investigation into Delta Air Lines as they struggle to return to normal operations after last week’s technical problems.

Delta Air Lines canceled more than 400 flights by 8:30 a.m. ET Tuesday. The airline has canceled around 6,000 flights since Friday.

Buttigieg said in a post on X that the investigation was to “ensure the airline is following the law and taking care of its passengers.”

A faulty software update sent to millions of computers running Microsoft Windows grounded flights Friday. American and United dealt with cancellations and logistical headaches throughout the weekend, but by Monday were returning to normal levels of cancellations.

Delta’s CEO said Monday it could take “another couple days” before its issues were resolved. The company says its crew-scheduling program is not yet fixed and it’s causing logistical problems.

It’s leaving travelers stranded and frustrated. Buttigieg previously said he was in contact with Delta and reminded passengers they’re entitled to a refund if they don’t want to be rebooked.

July’s calamity is the latest black eye for the airline industry. As travel resumed after the pandemic in the summer of 2022, the airlines faced a reckoning of staffing and weather challenges. Then, in December 2022, Southwest Airlines melted down as software problems and weather led to thousands of travelers getting stranded or scrambling for rental cars after Christmas.

The planes themselves have also faced scrutiny, as industry titan Boeing has taken heat for mechanical issues big and small in its planes — most notably when a fuselage panel blew off an Alaska Airlines flight.