DENVER -- Theme park owner Six Flags said it could sell or even close six of its 30 properties across North America, including the amusement park in downtown Denver, Six Flags Elitch Gardens.

Shares of Six Flags Inc. dropped sharply on Friday when debt rating agencies lowered their outlooks on the amusement park operator after it said attendance and revenues have fallen.

Six Flags shares shed 22.5 percent in afternoon trading to $5.77. The company's shares haven't had such a large single-day drop since July 16, 2004, when the price fell by about 25 percent.

After the market closed on Thursday Six Flags' chief executive Mark Shapiro told analysts and investors in a conference call that revenue was down about 1 percent through June 18, attendance fell by 13 percent, and the company may fail to comply with certain bank credit covenants.

Standard & Poor's lowered its outlook on Six Flags to negative from stable on Friday. The rating agency cited weak attendance, higher than expected operating costs and a tight "cushion of compliance" with its debt agreements.

"The size and diversity of the company's regional theme parks only partially mitigates these concerns," said S&P credit analyst Hal F. Diamond.

S&P gives Six Flags a corporate credit rating of B-minus, which is six notches below investment grade. The negative outlook was applied because "the company's turnaround effort appears to be more challenging than Standard & Poor's had anticipated, requiring more investment and time." In its report, S&P anticipated the outlook would not change over the next year, unless it saw improvements in profitability, debt and cash flow.

Six Flags is saddled with $2.1 billion in debt.

It's citing lower park attendance and rising costs, but said those who are coming to the parks are spending more.

President and CEO Mark Shapiro said the attendance drop was expected because the company is no longer deeply discounting season passes. It has been targeting families as consumers, rather than teenagers.

Shapiro said the New York-based company wants to reduce its reliance nationwide on teenage customers because they cause security problems and don't spend much money. The company, in evaluating which parks could be sold, also examined which ones sit on valuable real estate, he said.

The five other properties being examined for possible changes include Six Flags Darien Lake outside Buffalo, N.Y., Six Flags Waterworld in Concord, Calif., Wild Waves and Enchanted Village outside Seattle, Six Flags Splashtown in Houston and Six Flags Magic Mountain and Hurricane Harbor near Los Angeles.

Shapiro said Six Flags could invest in Magic Mountain to turn it into a family-style park, but noted that Southern California is packed with family-themed attractions including the two Disney parks in Anaheim, Knott's Berry Farm in Buena Park and Universal Studios Hollywood.

During a call with investors on Thursday, Shapiro said that Magic Mountain is a sale candidate partly because of its rowdy teenage atmosphere. The property also sits on 250 acres north of Los Angeles and would likely attract real estate developers.

"When Mark Shapiro took over, he promised some bold action, and this is bold," said Paul Ruben, the North America editor of Park World magazine. "I was surprised by his selection of parks. Magic Mountain in California, for example, is one of their flagship parks. And for that to go on the auction block was shocking."

"I think it might be a real estate play," said John Robinett, a senior vice president at Economics Research Associates. "Why would sell one of your flagship properties?"

The stock closed Thursday at $7.45. Over the past 52 weeks, the stock has ranged from $4.30 to $11.93.

Six Flags is the world's second-largest theme-park operator behind Disney.

Elitch Gardens moved from northwest Denver to downtown Denver in 1995.