WASHINGTON — The U.S. government says Standard & Poor’s knowingly inflated its ratings on risky mortgage investments that helped trigger the 2008 financial crisis.

The credit rating agency gave high marks to mortgage-backed securities because it wanted to earn more business from the banks that issued the investments, the Justice Department alleges in civil charges filed in federal court in Los Angeles.

The government is demanding S&P pay at least $5 billion in penalties.

The case is the government’s first major action against one of the credit rating agencies that stamped their approval on Wall Street’s soon-to-implode mortgage bundles. It marks a milestone for the Justice Department, which has long been criticized for failing to act aggressively against the companies that contributed to the crisis.

S&P, a unit of New York-based McGraw-Hill, called the lawsuit “meritless.”

“Hindsight is no basis to take legal action against the good-faith opinions of professionals,” the company said in a statement. “Claims that we deliberately kept ratings high when we knew they should be lower are simply not true.”

According to the lawsuit, S&P knew that home prices were falling and that borrowers were having trouble repaying loans. Yet these realities weren’t reflected in the safe ratings S&P gave to complex real-estate investments known as mortgage-backed securities and collateralized debt obligations.

At least one S&P executive who had raised concerns about the company’s proposed methods for rating investments was ignored.