By Basil Katz

NEW YORK (Reuters) - A man convicted over a plot to blow up Los Angeles airport who later cooperated with the government was sent back to prison on Friday for trying to purchase an AK-47 rifle and lying to authorities in violation of his probation.

Abdelghani Meskini, 42, an Algerian, pleaded guilty in 2001 to participating in the al Qaeda-sponsored "Millennium plot" to blow up Los Angeles International Airport.

In exchange for testifying against his co-defendants, he was given a more lenient sentence and freed in 2005.

In a ruling unsealed ahead of Friday's sentencing of Meskini -- to 31 additional months in jail -- U.S. District Judge John Keenan said the government was partly to blame for Meskini's drift back into trouble.

After he was freed, Meskini held a job at an Atlanta, Georgia, housing complex Keenan called a "hotbed of criminal activity," adding that U.S. law enforcement officials never suggested he not work there.

"Incredibly," the judge said, probation officers approved Meskini's job at the properties "where narcotics sales and prostitution occurred openly and persistently."

At the housing complex Meskini would collect rent and conduct maintenance. He was arrested by immigration authorities in November and transferred to Bureau of Prisons custody four months later.

Meskini's attorney, Mark deMarco, argued his client should be sentenced to time served and said the U.S. government was partly to blame for "depositing him in this hotbed ... putting him in a situation where the influences around him perhaps led him to his conduct, which is no excuse."

At an earlier hearing, prostitute and admitted crack cocaine user Crystal Roughton testified Meskini had asked her to help him purchase an AK-47 rifle.

She said Meskini helped her place escort service ads online and lent her money. She also testified she had seen Meskini in possession of a handgun, a probation violation.

Assistant U.S. Attorney Michael Rosensaft argued at another hearing that Meskini "had no interest in living a law-abiding life" and was becoming increasingly radicalized.

But deMarco said his client was "thrust into a hornet's nest" and "association with criminals was part of his job."

(Editing by Ellen Wulfhorst and Jerry Norton)