The two companies are in preliminary discussions for a deal that could be worth up to $6 billion
Calgary-based AltaGas did not name the third party, adding in its statement that no agreement has been reached and there is no assurance that these discussions will continue or that any transaction will be agreed upon.
Washington, D.C.-based WGL Holdings is the parent company of natural-gas utility Washington Gas, which supplies the U.S. capital region.
The Wall Street Journal, citing anonymous sources, said a deal could be announced this month but that regulatory or political pushback could be obstacles to any agreement.
AltaGas closed down 43 cents, or 1.26 per cent, at $33.65 after being halted earlier in the trading day with a market value of $5.58 billion.
WGL Holdings closed up US$4.50 or 5.94 per cent on the day for a market value of US$4.15 billion.