Authorities have not discovered any stowaways after a thorough search of a ship docked in New Jersey that the Coast Guard became suspicious of after they heard what sounded like knocks during a routine inspection.

Officials examined about 450 out of the 2,000 containers aboard the 850-foot cargo ship, which docked early Wednesday at Port Newark, one of the nation's busiest ports. The vessel was released for normal offloading, NBCNewYork.com reported.

Federal agents descended on the Cyprus-flagged Ville d'Aquarius after overnight inspectors heard noises "consistent with the sounds of people inside" coming from a cargo container below deck, Coast Guard spokesman Charles Rowe told NBCNewYork.com on Wednesday.

The search, which lasted more than 24 hours as authorities brought container after container onto the pier for closer inspection,ended around 8:45 a.m. ET, NBCNewYork.com said.

Michael Ward, New Jersey's top FBI official, said the response was appropriate given the port's vulnerability. It is considered a prime potential target for terrorists, according to NBCNewYork.com.

"You're going to get a response like this any time you have these types of facts," Ward said. "It was an appropriate response which we did out of an abundance of caution."

The Ville d'Aquarius originated in the United Arab Emirates on May 30, making stops in Pakistan, India, and Egypt before arriving in the Ambrose Channel, the main shipping channel for the Port of New York and New Jersey. It was there that Coast Guard officials believed they heard faint knocking during a 3 a.m. inspection.

The guardsmen knocked on a container several times, and heard several knocks back, Drew Barry, who piloted the ship to Port Newark for further inspection, told NorthJersey.com.

“Then they did it again,” Barry told NorthJersey.com. “They were pretty sure there was someone in there.”

Authorities from numerous agencies began searching the ship Wednesday morning. Shipping containers are typically steel boxes, about 8 feet wide, 8 to 10 feet high, and either 20 or 40 feet long, with hardly any ventilation, NBCNewYork.com said.

Ambulances lined up on the dock, ready to provide medical attention should any stowaways be found.

As the search dragged on, the large number of emergency personnel dwindled.

Coast Guard spokesman Rowe told NBCNewYork.com it took about eight minutes to check each container.

The ship's manifest said the container in question was carrying machine parts to be unloaded in Norfolk, Va.

Officials say they get stowaways in New York harbors about six times a year, NBCNewYork.com reported.

The Associated Press contributed to this report.

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AP Photo/Julio Cortez

A police official stands near the entrance to a terminal at Port Newark in Newark, N.J., on Wednesday as Immigration and Customs officials investigate reports of stowaways.

Updated at 7 p.m. ET: Immigration agents were called to Port Newark in New Jersey Wednesday morning amid reports that a ship docked there has multiple stowaways aboard. None were found by Wednesday evening.

Inspectors first became suspicious when they heard knocking and other noises "consistent with the sounds of people inside" coming from a cargo container below deck while the ship was anchored in the Ambrose Channel outside the Port of New York and New Jersey, Coast Guard spokesman Charles Rowe told NBCNewYork.com.

After hearing the noises during the routine overnight inspection, Coast Guard officials stayed aboard the Ville d'Aquarius, which had ports of call in Pakistan, Egypt, and India before its arrival, as it docked in Newark this morning, reported NorthJersey.com.

The container is believed to have been put on the ship in one of two ports in India -- either Mundra or Nahva Sheva -- before the ship left India on June 7, Rowe told NBCNewYork.com. The ship's last port before the United States was in Egypt on June 15.

The ship's manifest said the container was carrying machine parts to be unloaded in Norfolk, Va.

The Ville d'Aquarius is registered in Cyprus, and its current voyage originated in the United Arab Emirates. Initial reports had stated the ship started out in Pakistan.

NBC chopper video captured federal officials swarming around the New Jersey dock to investigate the vessel. More than a dozen ambulances also lined up in the morning, but as the day wore on with no findings other than cargo, emergency personnel started dispersing.

Details about the number of alleged stowaways were not immediately available.

"If there are people or other material, and we don't know what they are, we are simply covering all the bases," Rowe told New Jersey's Star-Ledger.

An official told NBCNewYork.com "it will take a significant amount of time to reach the container." 

Cargo containers were being brought onto the pier for examination. By midday, about 40 containers had been inspected among the approximately 2,000 on board.

Wednesday evening, officials with the Department of Homeland Security said they had inspected about one-third of the containers and no stowaways had been found. The search was expected to continue overnight, they said.

Officials say they get stowaways in New York harbors about six times a year, NBCNewYork.com reported.

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