Image caption Texas has the highest execution rate in the US with 482 prisoners killed since 1976

The US state of Texas is likely to have executed an innocent man due to careless handling of the case, a report by US law students claims.

Carlos de Luna was killed by lethal injection in 1989 for the brutal murder of a single mother six years earlier.

Right up until his execution, de Luna maintained a fellow Hispanic called Carlos Hernandez was the real culprit.

The Columbia University report backs his claim. It says "shoddy police work" probably led to the wrong man dying.

Under the supervision of Professor James Liebman, 12 students spent five years painstakingly dissecting the 1983 murder of Wanda Lopez in the Texan city of Corpus Christi. The petrol station clerk was stabbed in the chest with a lock-blade knife.

The students combed through endless police files and crime scene footage, and interviewed more than 100 witnesses, including detectives who had worked on the case.

'I know who did it'

The result is a 436-page book entitled Los Tocayos Carlos: An Anatomy of a Wrongful Execution, which, according to Professor Liebman, reveals an "emblematic" failure of the legal system.

The same factors that sent de Luna to his death continue to put innocent people at risk of execution today Professor James Liebman, Columbia University

"It looked like a common case, but we found that there was a very serious claim of innocence," said Prof Liebman.

The study details how 40 minutes after Lopez's murder, officers found de Luna hiding underneath a pick-up truck near the crime scene and arrested him.

He was charged with capital murder despite contradictory eyewitness accounts - the killer was seen fleeing towards the north, while de Luna was caught in the east - and the absence of blood on his clothing.

The law students also found that police had failed to test the murder weapon and victim's nails for blood and tissue samples.

Most importantly, investigators ignored de Luna's insistence that the real perpetrator was another Hispanic man known as Carlos Hernandez.

"I didn't do it, but I know who did," de Luna is quoted in the study as repeatedly saying, adding that he saw Hernandez entering the service station.

'Spitting image'

At the trial, the prosecution dismissed Hernandez as a "phantom" of the defendant's imagination. The Columbia study, however, confirms Hernandez was a well-known dangerous criminal.

Grim statistics There have been 1,295 executions in the US since the Supreme Court reinstated capital punishment in 1976

So far 482 prisoners have been killed in Texas, making it the state with highest execution rate

"Carlos Hernandez was always involved in violent crimes, and his weapon of choice was the knife," retired Corpus Christi detective Eddie Garza said in a video testimonial included in the report.

"My informants told me Hernandez had got someone else to take the fall [for Lopez's murder]. I contacted the detective in charge of the case. He said they had enough evidence linking Carlos de Luna to the crime so I backed away from it."

In essence, de Luna was executed because he looked the "spitting image" of the actual killer and officers handled the case with "nonchalance", said Prof Liebman.

"Everything that could go wrong in a death penalty case did go wrong for de Luna.

"Sadly, his story is not unique. The same factors that sent de Luna to his death - faulty eyewitness testimony, shoddy legal representation, and prosecutorial misfeasance - continue to put innocent people at risk of execution today."

Carlos Hernandez died of natural causes in a Texas prison in May 1999, having been jailed for assaulting a neighbour with a 9in (23cm) knife.