From the article at TV.com:
Fans react with anger at Sasha Alexander's NCIS death
by Lila Holland TV.com Staff Writer 06/10/05 05:19 PM
Viewers knew that one of the main characters on NCIS would die in this season's last episode, but the identity of that person remained one of television's best-kept secrets until the finale aired.
When Special Agent Kaitlin Todd (Sasha Alexander) was killed by a gunshot from double agent Ari, fans reacted with shock and rage. They flooded Internet message boards with angry postings directed at series creator Don Bellisario, who they felt was responsible for killing off one of the most important characters on NCIS. The verbal violence was only subdued when fans realized that Alexander had not been fired from NCIS but had in fact left the show of her own volition.
"This was the first time she went over a year on a show," says Bellisario of Alexander, "and she just didn't think she had the stamina to do it."
Alexander will be replaced with a new female lead character, who Bellisario hopes will be quite different from the conservative Agent Todd. "I want to go for a European or Australian girl who is very comfortable with her femininity and sexuality," he says.