CHICAGO (CBS) — Southwest Airlines canceled more than 1,800 flights nationwide on Sunday, blaming the weather and air traffic control issues, but sources tell CBS News a Southwest staffing shortfall this weekend was a factor in why so many of their flights were grounded.

There have been even more cancellations for Southwest on Monday, with nearly 40 flights at Midway International Airport scrapped as of 11 a.m.

READ MORE: City Hall, Chicago FOP File Dueling Lawsuits In Heated Standoff Over Vaccine Mandate

Meantime, the Federal Aviation Administration is directly countering Southwest’s blame game against air traffic controllers, saying that staffing shortage ended Friday.

Now many are wondering if this disruption is a glimpse of what’s to come as we approach Thanksgiving and holiday travel season.

Both the pilots and flight attendants unions say there are no official, or unofficial job actions.

READ MORE: Shots Fired At Police During Foot Chase In West Garfield Park

The pilots union has objected to the airline’s recent announcement that it would mandate COVID-19 vaccinations.

“We can say with confidence that our pilots are not participating in any official or unofficial job actions,” the pilots’ union said in the statement.

Passengers were confused about the weather cancellations on Sunday and some suggested that Southwest employees were calling in sick to protest the vaccine mandate.

In a statement, the airline said there was no “sick out” this weekend:

MORE NEWS: Illinois Democrats Look To Increase Edge With New Congressional Maps

We experienced weather challenges in our Florida airports at the beginning of the weekend, challenges that were compounded by unexpected air traffic control issues in the same region, triggering delays and prompting significant cancellations for us beginning Friday evening. We’ve continued diligent work throughout the weekend to reset our operation with a focus on getting aircraft and crews repositioned to take care of our customers.

CBS 2 Chicago Staff