When a US senator tried to visit a government shelter for migrant children, his trip ended not with a tour of the facility, but with a visit from police.

Senator Jeff Merkley attempted to visit a shelter in Brownsville, Texas this week, in the wake of a new Trump administration policy encouraging the separation of immigrant children from their parents at the border.

The shelter in question – which Mr Merkley said was housing children "torn away from their parents" – refused to arrange a tour for him when he called ahead. So the senator stopped by in person, with a live-stream camera team in tow.

“I think it’s unacceptable that a member of Congress is not being admitted to see what is happening to children whose families are applying for asylum,” Mr Merkley told the cameras. “...So I decided to come out here and go up to the door and explain why I’m here, and ask to be let in.”

But when he arrived at the windowless, locked doors of the shelter, Mr Merkley was told that he could not come inside.

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Instead, the senator stood outside for nearly 15 minutes before a manager came out to meet him. The manager refused to provide a statement, directing Mr Merkley to the Department of Health and Human Services (HHS), which oversees refugee resettlement.

At the same time, several police cars pulled up outside the shelter. The officers took down the senator’s name and date of birth and encouraged him to leave the property.

“I was barred entry. Asked repeatedly to speak to a supervisor – he finally came out and said he can’t tell us anything. Police were called on us,” Mr Merkley tweeted later.

“Children should never be ripped from their families & held in secretive detention centres," Mr Merkley added.

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The second half of the tweet was a reference to the Trump administration's new "zero tolerance" policy on illegal immigration. The new policy requires officials to prosecute anyone caught crossing the border illegally – even if they arrive with their children. While their parents are prosecuted, the children are separated and housed in shelters like the one Mr Merkley visited.

The shelter in question is operated by Southwest Key Programmes, a national nonprofit that subcontracts with HHS. Many of the children housed there came to the US unaccompanied, and are living in the shelter until they can be placed with sponsor families. ​

​HHS did not answer questions regarding how many children at this shelter arrived in the US alone, and how many had been separated from their parents.

In a statement to The Independent, a spokesperson said Mr Merkley and his team were denied access to the shelter “for the safety, security and dignity of the children being cared for there”.

“No one who arrives unannounced at one of our shelters demanding access to the children in our care will be permitted, even those claiming to be US Senators,” the spokesperson said. “Senator Merkley should respect the UAC program and engage in the appropriate processes, as many of his colleagues have done before him, to visit ORR facilities.”

It is unclear how many children have been separated from their parents since the Trump administration started implementing its new policy. A New York Times analysis found more than 700 children had been separated from adults claiming to be their parents between October 2017 and April of this year, before the policy started.