The crack FBI missing-persons team on Without a Trace may soon be missing one of their own. Vivian Johnson, the no-nonsense investigator played by Marianne Jean-Baptiste, might be killed off by season's end.

Earlier this year, Vivian was diagnosed with a serious heart condition called hypertrophic cardiomyopathy, which causes a thickening of the heart muscle and can lead to sudden death. The problem becomes critical when Vivian collapses on the side of the road in tonight's episode (10 pm/ET on CBS).

From then on, says series creator Hank Steinberg, "she will be home-ridden and coping with her future while [the squad] copes with the idea of not having her around." They may have to get used to that idea permanently.

"By the end of the season, she'll be angling toward surgery," hints Steinberg, "and potentially a life-and-death situation."

The story arc came about after Jean-Baptiste met with Steinberg early in the season to ask for meatier story lines. "It got to a point where not a lot was happening with my character," says the Oscar-nominated star of Secrets & Lies. "People would complain that they didn't know a lot about her."

As for the other characters, Steinberg promises lots of amazing twists. Look for Danny (Enrique Murciano) to once again have to help out his troubled brother, while "fireworks" between Martin (Eric Close) and Samantha (Poppy Montgomery) cause a rift in their relationship.

Also, look for some "heat" between Sam and her boss and former lover, Jack Malone (Anthony La Paglia), and for the finally divorced Jack — who continues to wrestle with his demons — to start up a new flirtation. Steinberg reveals, "There's a big surprise coming that avid fans will love."