SAN ONOFRE, Calif. -- A Customs and Border Protection helicopter that crash-landed on San Onofre Beach was partially taken apart Friday so it could be removed, according to immigration officials.

The three federal agents aboard the McDonnell Douglas 600 suffered minor injuries when the aircraft came down in the surf near the San Onofre Nuclear Generating Station during a routine patrol flight about 7:30 p.m. Thursday, according to CBP officials.

The pilot reported engine trouble before making a rough emergency landing, the federal agency reported.

Witnesses heard a loud sound and saw sparks shooting out of the back of the helicopter as it was heading south above the coast. The aircraft then caught fire and came down, they said.

The agents, who are members of the CBP Office of Air and Marine, were taken to a hospital.

Crews partially disassembled the banged-up craft before loading it onto a truck and removing it from the beach late Friday morning, said investigator Jim Struhsaker of the National Transportation Safety Board, which is in charge of the probe.

The copter will likely be stored at North Island in Coronado while officials from the NTSB and Federal Aviation Administration scrutinize it to determine what went wrong, according to Struhsaker.

"It's really too early," to speculate about what caused the helicopter to malfunction, Struhsaker said Friday afternoon.

The aircraft was en route from Long Beach to Brown Field airport in Otay Mesa when the crash landing occurred.