The Justice Department is investigating whether U.S. airlines colluded on expansion plans, amid concerns from consumer advocates and politicians that the industry is trying to extend its recent run of prosperity by controlling capacity to keep airfares high.

A Justice Department spokeswoman said the department is looking at “possible unlawful coordination” among some airlines. She declined to elaborate or identify the airlines being examined. The investigation began about two months ago, a person familiar with the matter said.

The four largest U.S. airlines— American Airlines Group Inc., United Continental Holdings Inc., Delta Air Lines Inc., and Southwest Airlines Co. —confirmed that they are part of the probe and said they are cooperating.

JetBlue Airways Corp. and Alaska Air Group Inc. said the Justice Department hadn’t contacted them as of Wednesday.

The Justice Department has requested documents from airlines related to expansions, as well as documents related to meetings with analysts or officials at other airlines, according to a copy of a letter it sent to one of the airlines, and which was reviewed by The Wall Street Journal.