SAN FRANCISCO (AP) — Small cracks have appeared in a new concrete spillway at Oroville Dam, a development state officials say was expected but an engineering expert says could lead to serious safety issues.

In a previously undisclosed October letter, federal regulators asked Department of Water Resources officials to explain the hairline cracks on the dam's new massive concrete flood-control chute, KQED radio of San Francisco reported Tuesday.

The Federal Energy Regulatory Commission also asked water officials what, if any, steps might be required to address the issue.

In February, authorities ordered nearly 200,000 people downstream of the dam to evacuate when both spillways suddenly began crumbling. The feared uncontrolled releases of water over the dam did not occur, and authorities allowed residents to return to their homes within days.

State officials say emergency and subsequent repairs of the 770-foot (235-meter) dam have so far cost at least $640 million but not all costs have been identified yet.

In their response to federal regulators, California water officials said in November that the state's efforts to build a more durable spillway caused the cracks, which were anticipated.

"The hairline cracks are a result of some of the design elements included to restrain the slabs and produce a robust and durable structure," the letter read, adding that the cracking "was anticipated and is not expected to affect the integrity of the slabs."

The evidence for and reasoning behind DWR's statements about the cause of the cracking is not available for independent assessment, the station reported.

University of California civil engineering professor Robert Bea, a veteran analyst of structure failures, said cracking in high-strength reinforced concrete structures is never expected.

The cracking "develops paths for water to reach the steel elements embedded in the concrete and accelerate corrosion," Bea wrote in an email. "Such corrosion was responsible for the degradation and ultimate failure of the steel reinforcing in parts of the original gated spillway."