Shares of Cal-Maine Foods, the largest U.S. egg producer, fell in after-hours trading Tuesday after the company acknowledged it is being investigated by the antitrust division of the U.S. Department of Justice.

Ridgeland, Mississippi-based Cal-Maine said it received notice of the investigation into egg price increases last month. Cal-Maine said it is cooperating with the investigation.

The company's shares fell more than 4% in after-hours trading.

Egg prices have hit record highs in recent months, largely due to a bird flu epidemic that has forced farmers to slaughter more than 166 million birds, mostly egg-laying chickens.

One dozen Grade A eggs cost an average of $5.90 in U.S. cities in February, up 10.4% from a year ago. That eclipsed January's record-high price of $4.95.

The egg price increases have put Cal-Maine, which provides around 20% of the nation's eggs, under increased scrutiny.

On Tuesday, Cal-Maine said its sales nearly doubled to $1.42 billion in its fiscal third quarter, which ended March 1. The company said that was primarily due to higher egg prices, which averaged $4.06 per dozen during the quarter, up from $2.25 per dozen a year ago.

Cal-Maine's sales fell short of Wall Street's forecast of $1.43 billion, according to analysts polled by FactSet.

Cal-Maine said it sold a record 331.4 million dozen-eggs in the third quarter, a 10% increase from the same period a year ago.

Cal-Maine said it made progress on mitigating the effects of bird flu, including increasing the number of layer hens and chicks hatched and recovering from the flu-related closure of facilities in Texas and Kansas. The company said its feed costs were also down during the quarter.

Cal-Maine said its third quarter net income more than tripled to $508.5 million compared to the same period a year ago. The profit, of $10.38 per share, also fell short of analysts' forecast of a $10.72 per-share profit.