MADRID, Spain — A serial killer sentenced to 440 years in jail for raping and strangling 16 elderly widows was murdered in prison, officials said today.

Two prisoners with makeshift knives attacked 44-year-old Jose Antonio Rodriguez Vega in the courtyard of Topas jail in western Salamanca province Thursday, two days after his transfer from another facility where inmates had also attacked him, prison director Jose Ignacio Bermudez said.

The Topas attackers said they'd acted because Rodriguez Vega was a rapist and under the code of honor among prisoners that made him a pariah, Bermudez said.

An unarmed prison guard tried to pull them off but couldn't. One kept stabbing Rodriguez Vega as he lay dead, he said.

"They told prison officials 'go away, this is our business,' " Bermudez said.

He denied reports the inmates had identified Rodriguez Vega from a crime show broadcast in February by the television affiliate of the newspaper El Mundo.

Rodriguez Vega was convicted in 1991 of raping and strangling 16 elderly women in the northern city of Santander over a space of 10 months in 1987 and 1988.

Vega allegedly got into their homes by posing as a television repairman or insurance salesman.

In Spain, prisoners can serve a maximum of 30 years in jail, and because of sentence reductions for work done in prison Rodriguez Vega had been due for release in 2008.