A stretch of the Mojave Desert has been transformed by hundreds of thousands of mirrors into the largest solar power plant of its type in the world, but the milestone is being met with criticism from environmental groups concerned about the effect of solar energy on desert wildlife.

The Ivanpah Solar Electric Generating System, sprawling across roughly 5 square miles of federal land near the California-Nevada border, formally opened Thursday after years of regulatory and legal tangles ranging from relocating tortoises to assessing the impact on plants.

The $2.2 billion complex of three generating units, owned by NRG Energy Inc., Google Inc. and BrightSource Energy, can produce nearly 400 megawatts — enough power for 140,000 homes. It began making electricity last year.

Google announced in 2011 that it would invest $168 million in the project. As part of its financing, BrightSource also lined up $1.6 billion in loans guaranteed by the Department of Energy.

Using technology known as solar-thermal, nearly 350,000 computer-controlled mirrors roughly the size of a garage door reflect sunlight to boilers atop 459-foot towers. The sun's power is used to heat water in the boilers' tubes and make steam, which drives turbines to create electricity.

The system, however, is coming under scrutiny from some environmentalists because of growing evidence the technology is scorching birds that fly through the intense heat surrounding the towers, which can reach 1,000 degrees Fahrenheit, The Wall Street Journal reported.

BrightSource reported finding dozens of dead birds at the plant over the past several months, with some having singed or burned feathers, according to federal biologists and documents filed with the California Energy Commission, the Journal reported. State and federal regulators are currently conducting a two-year study of the Ivanpah plant's effects on birds.

Meanwhile, U.S. Fish and Wildlife Service biologists are voicing concerns about BrightSource's plan for a second large-scale solar farm in Riverside County, which also has the potential to produce intense heat that could kill golden eagles and other protected wildlife.

"We're trying to figure out how big the problem is and what we can do to minimize bird mortalities," Eric Davis, assistant regional director for migratory birds at the agency's Sacramento office, told the Journal. "When you have new technologies, you don't know what the impacts are going to be."

Biologists investigating the deaths think some birds may have mistaken the shimmering mirrored desert area for a lake and became trapped on the ground after landing, the Journal reported.

Western Watersheds Project, an Idaho-based conservation nonprofit group, sued the federal government over its approval of the solar energy plant in 2011, arguing that the facility was approved without an adequate environmental assessment.

A California district court tossed the group's lawsuit in November 2012. Western Watersheds recently restated its pleas to the U.S. Ninth Circuit Court of Appeals, the International Business Times reported.

"We know solar projects are an issue as far as birds are concerned," Michael Connor, the group's California director, told the publication. "But they say, well, it’s not significant, so we’ll monitor it. So what happens now? You find lots of dead birds. Then what happens? Nothing."

Joe Desmond, a spokesman for BrightSource, told The Wall Street Journal the company is working to resolve the problem of bird deaths and plans to build more big solar plants in California and other states.

Western Watershed is continuing to push its lawsuit against federal agencies that reviewed the Ivanpah project. Connor said alternatives to the site were not considered and serious environmental impacts, including fragmenting the tortoise population, were ignored.

"Do we really need to have these giant plants first, or is it better to generate solar power on people's roofs, the place it's going to be used?" Connor asked.

Ivanpah can be seen as testing the balance between the need for cleaner power and the loss of fragile, open land. The California Energy Commission concluded that while the solar plant would impose "significant impacts on the environment ... the benefits the project would provide override those impacts."

Such disputes are likely to continue for years as more companies seek to develop solar, wind and geothermal plants on land treasured by environmentalists who also support the growth of renewable energy.

According to statistics compiled by the Energy Department, the solar industry employs more than 140,000 Americans at about 6,100 companies, with employment increasing nearly 20 percent since the fall of 2012.

The Associated Press contributed to this report.