WASHINGTON, United States (AFP) — Among the 3,000 counties that make up the United States, there is one in Texas where it’s best not to mess with the law, because justice is hard and mistakes are common.

Harris County has executed a record 125 people since the US Supreme Court overturned the death penalty in 1976.

“The best answer I know is that it’s a huge county — four million people — that’s very conservative, in an active death penalty state, and for a long time had a notoriously bloodthirsty DA,” Samuel Gross of the University of Michigan law school told AFP.

Home to the sprawling city of Houston, Harris County accounts for nine per cent of all modern US executions.

It has executed more people than that any of the 31 states which administer the death penalty, except for the state of Texas as a whole.

“There are four major reasons why Harris County has put so many people on death row: overzealous prosecutors, poor legal representation, racial bias, and the absence of a life without parole sentencing option until 2005,” said Robert Dunham, director of the Death Penalty information Center.

Studies have shown that Harris County prosecutors were three times more likely to seek the death penalty against African Americans than against white defendants between 1992 and 1999. Juries there were more than twice as likely to impose death sentences on African Americans during the same period.

Lawyers for Duane Buck, sentenced to die in Harris County in 1997, filed an appeal with the Supreme Court Thursday because an expert witness told jurors he posed a higher risk of recidivism because he was black.

Harris County also accounted for a third of the nation’s exoneration cases in 2015, according to a study Gross published Wednesday.

Many of those who spent years behind bars until they could prove their innocence include African Americans like Alfred Brown, who was arrested in 2003 at the age of 21 for a double homicide and sentenced to death in 2005.

Prosecutors suppressed phone records showing Brown was at his girlfriend’s home at the time of the crime and jailed his girlfriend on perjury charges until she agreed to testify against him, according to a series of columns which netted Houston Chronicle reporter Lisa Falkenberg a Pulitzer Prize.

Brown was released in June after 12 years behind bars.

The vast majority of exonerations are in drug cases where people caught up in the system are pressured to plead guilty because they have little hope of clearing their names.

“It’s shocking, but it is very common,” said Jim Cohen, a professor at Fordham Law School.

“They are pushed by the prosecutor and they are pushed by the defense attorney because the defence attorney is saying, ‘If you don’t plead guilty and you are convicted after trial, you’re going to get a much bigger sentence.’”

But the evidence which pushed them to confess was often flawed or even inadmissible.