After mistakenly thinking she saw something fall through the ice of the Charles River, a Leverett House senior called police last night, prompting seven police cars, four fire trucks, and a special ice rescue squad to rush to the scene about 5:30 p.m.

But within 20 minutes, two rescue squad members in inflatable yellow body suits crossed the frozen river and found no signs of any accident.

The witness, who asked not to be named, said she spotted from her Leverett Tower window a dark shape moving in the river between the Weeks and Anderson bridges and immediately notified Harvard Police.

"I saw a dark shape moving around and first I thought it was a bird or a dog. But there was some doubt that maybe there could be someone there," she said. "To be safe I called the police."

"We made as thorough a search as possible and didn't find anyone. We feel certain no one's there," said Deputy Chief William F. Murray.

A black marker protruding a couple feet out of the ice near the Boston side of the river might have been what the student saw, according to Murray.

The student said police arrived at the scene about three minutes after the call and fire and rescue trucks followed soon after. Murray explained that "we must get them out in the first five minutes [because] people are often brain-dead in about 10-15 minutes."

Murray said about six people fall through the Charles each winter. He could not recall when, if ever, a Harvard student had plunged into the icy river.

Last Saturday, an 11-year-old boy who fell through the ice near the Anderson Bridge was pulled to safety by a rescue team, Murray said. The boy suffered slight hypothermia.

Because it appears to be solidly frozen over, the Charles can be a temptation to cross-country skiers and children, the deputy chief said. But, he added, "the Charles doesn't freeze as well as a still pond, therefore it is especially dangerous."