WASHINGTON — The mother of US ambassador Chris Stevens, killed in last month's attack on a US mission in Libya, hit out Wednesday at the row over his death playing out in Washington, saying it was unproductive.

But Mary Commanday said she still did not know much about the circumstances of how her son died when the US consulate in Benghazi was attacked, and had had no update or communication with the White House.

In her first interview since Stevens was killed in the September 11 assault, Commanday told CBS television that she did not want to talk about the political firestorm his death has unleashed just weeks from the 2012 elections.

"I don't think it's productive to lay blame on people," she said.

Republicans have gone on the attack against the administration of President Barack Obama -- who is seeking re-election on November 6 -- accusing it of huge security failures and of trying to cover-up the circumstances of the events.

It led to a furious exchange in Tuesday's debate when Obama told his Republican challenger Mitt Romney it was "offensive" to suggest that his administration was playing politics or trying to mislead people.

Commanday described her son as a "cheerful, positive-thinking person" and said she had voiced concern to him about his safety.

"I asked him about it and I said, 'It's pretty dangerous over there, Chris.' He said... he had bodyguards and he trusted his bodyguards," Commanday said.

The family held a memorial service for Stevens this week in San Francisco, but are still not clear about how he died.

"I mean, you have to know something about something in order to have questions about it and I don't know enough really to ask questions," Commanday said.

She added that even knowing all the answers would not be an end. "Well, it doesn't bring him back, does it?" she asked.