Still no signs of stowaways aboard a cargo ship that arrived at the Port Newark Container Terminal early Wednesday morning.

As part of a routine inspection, Coast Guard officers heard knocking, consistent with human response, coming from one of the containers in the ship's hull.

The noises ended after two hours, but triggered a massive response effort, with dozens of emergency vehicles and ambulances lined up at the Port.

"We heard noises that would indicate it was a person, not just something knocking around" said Coast Guard spokesman Charles Rowe.

Large mechanical cranes began unloading containers from the ship, one by one. By midday Wednesday, all but one ambulance had quietly left the pier. By late evening, officials had inspected 150 of the 200 containers authorities believe could be carrying people. The ship has 2,000 containers altogether.

"I heard someone was banging on the if they were banging on the side, they need help" said Juan Nolan, a veteran dock worker at the Port in Newark.

"It's a highly unusual thing to find...its maybe happened three times...its not a regular thing" he said.

Shipping containers are steel boxes, usually 8 feet wide and 8- to 10-feet high and either 20- or 40-feet long, designed to withstand the rigors of the high seas and are strong enough to be stacked several high. They normally can be opened only from the outside. There's hardly any ventilation. Rowe said it was taking about eight minutes to check each container -- unloading it off the ship, opening it up and X-raying it if necessary.

"Its a very long process, but we are simply covering all our bases at this point" Rowe said.

The Department of Homeland Security, which was also involved in the investigation, said Wednesday night that its officers and agents were prepared to continue examining containers through the night.

The container ship, Ville D'Aquarius, which a manifest said was carrying machine parts to Norfolk, Va., was loaded in India, Rowe said. The ship began its voyage May 30 in the United Arab Emirates, then made one stop in Pakistan and two stops in India. Its last port before Newark was in Egypt.

Even if nothing is found, John Gomez, with Group Espada, a New Jersey based training group for law enforcement officers, said it does send a message to would-be copycats.

"It sends a powerful signal to anybody that would want to try something...that it may not be as easy as you think and that there are people out there trying to keep us safe and secure."

(The Associated Press contributed to this report)