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In 2018 thousands of families trying to enter the US from Mexico were separated at the border between the two countries.

This was part of US president Donald Trump's "zero tolerance" policy which targeted people who were trying to get into America from Mexico illegally.

The policy provoked widespread criticism from around the world and in June 2018 a US judge ordered that migrant children and their parents should be reunited.

About two-thirds of the parents have been deported back to the country they were born in.

However, the American Civil Liberties Union (which is an organisation working to protect people's rights) says that the parents of 545 children still cannot be found.

Building a wall along the US-Mexico border, to prevent people from entering the US illegally, was one of Donald Trump's key promises when he was campaigning to become US president in 2016.

It was an idea that divided the country, but it won Donald Trump many supporters.

In July 2019, the president was given the go-ahead to use $2.5 billion (£2bn) of military budget on a section of wall on the border.

The administration said it would be used to construct more than 100 miles of fencing.

Mr Trump, a Republican, declared an emergency in 2019, saying he needed $6.7bn to build the wall as a matter of national security. The cost of a barrier along the whole 2,000 miles (3,200km) of border is estimated to be $23bn.

Democrats say they are in favour of border security but the wall would be expensive and ineffective.

The U.S. Border Patrol says it has completed 321 miles (517 kilometres) of wall during the Trump administration, though almost all of that is replacing existing barriers.

In 2018 thousands of children were separated from their parents when they were detained by border guards as part of the US government's crackdown on illegal immigration.

If adults brought children with them, when they are arrested, those kids had to be taken into the care of the authorities.

They were transferred to government shelters or foster care, but there were also reports of children being kept in detention centres under terrible conditions.

At the time, President Donald Trump initially defended the policy, saying he "didn't want" to take children from their parents, but "when you prosecute the parents for coming in illegally - which should happen - you have to take the children away".

However he ended the policy a few months later.

Some families were separated in 2017, as part of the government's trial of their "zero tolerance" programme.

This meant that when a judge ordered that the families be reunited in 2018, the group from 2017 were not included.

It took until 2019 for a judge to order that families separated in 2017 be reunited. Of this group of 1,030 separated children, 485 have had their parents found but 545 still have not.

The American Civil Liberties Union (ACLU) and a team of lawyers have been searching for the missing parents but say the process has been delayed by the coronavirus pandemic.

Lee Gelernt from ACLU said "these families must be reunited" and that the government "must be held accountable".

Mr Gelernt added that the ALCU will continue looking for the families until each one is found.