Story highlights London dockworkers discovered 35 people inside a shipping container

One man has died, the rest treated for dehydration and hypothermia

Police say they are from the Indian subcontinent

Workers heard screaming and banging from one of the ship's 50 containers

Dockworkers at a major London port discovered 35 people inside a shipping container Saturday morning.

One of them, an adult male, was declared dead at the scene, according to Essex police Superintendent Trevor Roe.

Several others -- including children -- were said to have "significant health problems," according to a police statement.

Daniel Gore of England Ambulance Service said the patients -- all of whom were conscious when they were found -- are being treated for dehydration and hypothermia at three London-area hospitals.

None of their conditions are believed to be life-threatening, he said.

What caused the one death was not yet known. A homicide investigation is under way, according to Roe.

'Screaming and Banging'

Workers at Tilbury Docks made the discovery after "screaming and banging was heard from inside the container," Roe said.

The stowaways are from the Indian subcontinent, said Roe, but it was not immediately clear if that is where their maritime journey originated, or when it did.

What is known is that the container they were in arrived to Tilbury -- located on the River Thames about 30 miles east of London -- aboard a ship owned by P&O Ferries at 6 a.m. (1 a.m. ET) from the Belgian port city of Zeebrugge after a nearly nine-hour trip.

Company spokeswoman Natalie Hardy told CNN that P&O Ferries operates two such trips a day between London and Zeebrugge.

She said P&O Ferries does not inspect the contents of the containers it transports as a matter of company policy. Such checks, according to Hardy, are the responsibility of the UK Border Agency.

There were 50 containers on the ship, according to Roe. He said they would all be thoroughly searched.

British authorities blasted the "criminal networks" engaged in human trafficking.

"Today's tragic incident at Tilbury is a reminder of the often devastating human consequences of illegal migration," Immigration and Security Minister James Brokenshire said in a statement. "This incident is now a criminal investigation. Border Force officers are fully engaged with Essex police and the Tilbury port authorities as the necessary steps are taken towards bringing those responsible to justice."