Stowaways are suspected to be in a container loaded in India that has docked in Newark, N.J., the coast guard said.
By midday, federal authorities boarded the ship, The Ville D'Aquarius, and had inspected 80 of the 200 containers they believe could be carrying stoaways. Federal and local law enforcement officials, as well as ambulances are on the scene.
Coast Guard spokesman Charles Rowe says it's been taking about eight minutes to check each container. It involves taking it off the ship, opening it up and X-raying it, if necessary.
A boarding party heard sounds consistent with people coming from the ship, according to Rowe. The team, however, could not pinpoint where the sound was coming from. The return knocks ended after about two hours.
Drew Barry, of the Sandy Hook Pilots Association, said he boarded the vessel about 20 miles offshore to help pilot it into port.
"There are at least 30 to 40 containers on top of the hatch cover, and I don't know how many more below it," Barry said. "If there are people down there, with no food and water for days, they're probably pretty desperate by now."
Customs and Border Patrol agents met the 850-foot-long vessel at Port Newark on Wednesday morning.
The ship began its voyage at the end of May in the United Arab Emirates, making one stop in Pakistan, two stops in India and make its last stop in Egypt on June 14 before heading to Newark.
Tracie Hunte and the Associated Press contributed reporting