ARECIBO, Puerto Rico — A huge, already damaged radio telescope in Puerto Rico that has played a key role in astronomical discoveries for more than half a century completely collapsed on Tuesday.

The telescope's 900-ton receiver platform and the Gregorian dome — a structure as tall as a four-story building that houses secondary reflectors — fell onto the northern portion of the vast reflector dish more than 400 feet below.

The U.S. National Science Foundation had earlier announced that the Arecibo Observatory would be closed. An auxiliary cable snapped in August, causing a 100-foot gash on the 1,000-foot-wide dish and damaged the receiver platform that hung above it. Then a main cable broke in early November.

The collapse at 7:56 a.m. on Tuesday wasn't a surprise because many of the wires in the thick cables holding the structure snapped over the weekend, Ángel Vázquez, the telescope's director of operations, told The Associated Press.

"It was a snowball effect," he said. "There was no way to stop it.... It was too much for the old girl to take."

The collapse stunned many scientists who had relied on what was until recently the largest radio telescope in the world.

"It sounded like a rumble. I knew exactly what it was," said Jonathan Friedman, who worked for 26 years as a senior research associate at the observatory and still lives near it. "I was screaming. Personally, I was out of control. ... I don't have words to express it. It's a very deep, terrible feeling."

Friedman ran up a small hill near his home and confirmed his suspicions: A cloud of dust hung in the air where the structure once stood, demolishing hopes held by some scientists that the telescope could somehow be repaired.

Vázquez, the telescope operations director, said that it was extremely difficult to say whether anything could have been done to prevent the damage that occurred after the first cable snapped in August.

"The maintenance was kept up as best as we could," he said. "(The National Science Foundation) did the best that they could with what they have."

Installing a new telescope would cost up to $350 million, money the NSF doesn't have, he said, adding it would have to come from U.S. Congress.

Scientists worldwide had been petitioning U.S. officials and others to reverse the NSF's decision to close the observatory. The NSF said at the time that it intended to eventually reopen the visitor center and restore operations at the observatory's remaining assets, including its two LIDAR facilities used for upper atmospheric and ionospheric research, including analyzing cloud cover and precipitation data. The LIDAR facilities are still operational, along with a 12-meter telescope and a photometer used to study photons in the atmosphere, Vázquez said.

The telescope was built in the 1960s with money from the Defense Department amid a push to develop anti-ballistic missile defenses. It had endured hurricanes, tropical humidity and a recent string of earthquakes in its 57 years of operation.

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