United Airlines planes sit on the tarmac at San Francisco International Airport on July 8, 2015. Getty Images/Justin Sullivan

Shares of United Continental Holdings Inc. (NYSE:UAL) fell 3 percent Friday after CEO Oscar Munoz suffered a heart attack a day earlier, the Wall Street Journal reported, citing a person familiar with the matter. The 56-year-old executive is being treated at a Chicago hospital.

United directors are “waiting to hear back from his family and his doctors" regarding further details about the heart attack, the Journal said. They hope to know within the next 24 hours whether the airline will need to appoint an interim leader, the Journal reported, citing a source.

We have been informed by Oscar’s family that he was admitted to the hospital on Thursday and we will provide further details as appropriate. In the meantime, we are continuing to operate normally. Our thoughts and prayers are with his family and we are respecting their privacy, the airline said in a statement Friday.

Oscar Munoz Overview | FindTheCompany

Following the announcement, shares of the second-largest U.S. airliner fell more than 3 percent, trading as low as $55.05.

Munoz was recently named president and chief executive officer of United Airlines on Sept. 8 after Jeff Smisek stepped down amid a federal investigation into the airline’s relationship with the former chairman of the Port Authority of New York and New Jersey.

The company is scheduled to report its latest quarterly results next Thursday.

United Continental Holdings is forecast to report third-quarter earnings of $1.5 billion, or $4.38 per share, on revenue of $10.3 billion, according to analysts polled by Thomson Reuters. That compares with a profit of $924 million, or $2.37 per share, on sales of $10.6 billion for the same time last year.

In July, United Continental cut its domestic capacity growth outlook for the year amid soft revenue.

The carrier now expects domestic capacity, or the expansion in the supply of available U.S. seats this year, to grow in the range of 0 percent to 1 percent, down from an earlier forecast in April had called for growth of 0.2 percent to 1.2 percent.