The world's richest art institution knowingly bought scores of archaeological treasures looted from Italy, it has been alleged.

Despite being warned as far back as 1985 that dealers were selling stolen goods, the Getty Museum in Los Angeles continued to buy them. The practice continued for so long that, according to the museum's internal review, almost half the masterpieces in its antiquities collection are likely to have been acquired illegally.

New evidence of the scale of what Italy is calling "the Getty scandal" emerged yesterday, painting a picture of the fabulously wealthy institution riding roughshod over a ban on taking Italy's historic treasures out of the country.

According to the Los Angeles Times, which has obtained hundreds of pages of memos, purchase agreements and correspondence records from the museum in Malibu, high-ranking staff were complicit or simply turned a blind eye to the plundering of the priceless antiquities.

The investigation makes several main claims:

· that Getty officials spent $10.2m (£5.7m) in 1985 to acquire three objects taken from ruins near Naples, despite being warned that the purchase was in clear defiance of Italy's "cultural patrimony" laws, which state that all artifacts discovered after 1902 are government property;

· that the museum purchased an ancient urn for $42,000 despite being told that the Italian police were looking for it;

· that it spent $18m in 1988 on a statue of Aphrodite dating back to 400BC which was probably the centrepiece of a Greek temple in southern Italy, even though officials were suspicious of the dealer's explanation about where it came from.

The newspaper also prints extracts from the resignation letter in 1986 of an acting curator who talked about problems in the antiquities department and warned that the museum's "cultural avarice" would some day lead to demands from foreign governments for the return of looted artifacts.

Italian authorities are demanding the return of 42 objects in the Getty collection that they believe were stolen, including a 5ft marble statue of the Greek god Apollo unearthed in southern Italy and said to be more than 2,000 years old.

The museum's own lawyers, however, have found even more examples of artifacts of dubious provenance, according to the report. They counted 82 artworks as being purchases from dealers and galleries under investigation by Italian officials, including 54 of 104 artistic treasures described by the Getty as masterpieces.

The Getty's former antiquities curator Marion True and antiquities dealer Robert E Hecht Jnr have been charged by the Italian authorities with conspiring to export illegally excavated treasures. They have denied the charges and a trial is scheduled in Rome in November. Another dealer, Giacomo Medici, received a 10-year prison term but remains free pending an appeal.