Berlin is investigating whether US immigration policy has changed, after a German national who is a permanent US resident was detained and “violently interrogated” by US border officials.

Fabian Schmidt, 34, is being held at a detention centre in Rhode Island after attempting to return to his home in New Hampshire after a trip to Luxembourg.

His mother, Astrid Senior, told the Boston-headquartered public broadcaster WGBH news that her son had been “violently interrogated” at Boston’s Logan airport for hours, stripped naked and put in a cold shower by two officials.

A German government spokesperson told Euronews it was aware of the case and those of two other German citizens who were detained when attempting to enter the US and who have subsequently returned to Germany.

“We have recently become aware of three cases in which German citizens were unable to enter the USA and were detained for deportation when they entered the country,” the spokesperson, Sebastian Fischer, said.

Two of the three cases had been resolved, Fischer said, adding that the German government was “monitoring the situation” and liaising with other EU countries to assess if these were isolated cases or representative of “a change in American immigration policy”.

Senior told the US broadcaster that immigration agents had put pressure on her son to give up his green card. He had been placed on a mat in a bright room at the airport with little food or water, and denied access to his medication for anxiety and depression.

Schmidt, an electrical engineer, had been returning to the US on 7 March. His mother did not hear from him until 11 March, when he was transferred to hospital after collapsing.

She told NBC news she had no idea why her son had been detained, saying: “I feel helpless, absolutely helpless.”

Schmidt and his mother moved to the US from Germany in 2007 and got permanent resident cards the following year. He renewed his legal permanent residence status last year and has no active legal issues, his mother said, although she added that he had faced misdemeanour charges about 10 years ago.

The US Customs and Border Protection service told US media the claims about his case were “blatantly false”, without saying which points it disputed.

The CBP sent the Guardian the same statement, without further explanation. The statement also said: “When an individual is found with drug-related charges and tries to reenter the country, officers will take proper action.”

The German foreign ministry said its consulate in Boston was providing support.

“It is clear that we expect from our partners, as well as from all other countries in the world, that prison conditions comply with international human rights standards and that detainees are treated accordingly,” Fischer said.

The case is the latest of a foreign-born national being detained and allegedly held in brutal conditions by US border agents, after Donald Trump’s crackdown on immigration.

The two other German nationals affected were Jessica Brösche, a 29-year-old tattoo artist from Berlin, and Lucas Sielaff, 25, from Saxony-Anhalt. Both had valid tourist visas for the US and have now returned to Germany.

According to Sielaff’s fiancee, Lennon Tyler, he was detained for two weeks after incorrectly answering a question about where he lived owing to his poor grasp of English. Sielaff had said Las Vegas, where he was staying with Tyler, when he should have said Germany, where he permanently resides, she said.

Brösche – who was travelling with tattoo equipment – was detained for more than six weeks, including nine days in solitary confinement, her friends said – a claim the company that owns the detention centre denies.

Meanwhile, a Canadian actor said she was detained for nearly two weeks in “inhuman conditions” over an incomplete visa, after attempting to cross the border from Mexico. Jasmine Mooney was held in a single concrete cell with no natural light, fluorescent lights that were never turned off, with no blankets and limited bathroom facilities, her mother said.

And a British woman was detained in “horrendous conditions” for three weeks, her family said, after a visa mix-up.

Rebecca Burke is now reportedly home in the UK. She was arrested by US border officials after attempting to cross into Canada and being sent back to the US.

She had been planning to stay with a host family in Canada, where she would carry out domestic chores in exchange for accommodation, but was told by the Canadians she should have applied for a working visa instead of a tourist visa.

Burke had previously stayed with a host family in Portland, Oregon under a similar arrangement after spending time sightseeing in New York City, where she arrived from the UK at the start of the year.