Prosecutors have been unable to determine whether a mix of prescription drugs intoxicated a St. Amant woman and impaired her ability to drive when she drove off Stringer Bridge Road into Black Bayou one night in February 2012, drowning her two daughters trapped in the car.

Twenty-Third Judicial District Attorney Ricky Babin said that because of that failure and other investigative dead ends, his office is unable to move forward with a criminal case against Raelyn M. Encalade, 31, in the deaths of Faith Castilaw, 9, and Patience Lobell, 7.

“We’re at a dead end. I would still do something if I got some information,” said Babin, who is district attorney for Ascension, Assumption and St. James parishes. “I’ve got to have something.”

Babin said the criminal case, however, has not been formally closed. The statute of limitations allows charges to be filed for six years from the crash.

Encalade, who lives near the scene of the crash, was driving back home on Stringer Bridge, also known as La. 935, from a convenience store about 9:41 p.m. Feb. 21, 2012, troopers have said.

Encalade, who was driving her daughters and two other young girls as passengers, missed a curve in Stringer Bridge, and the 2003 Mercury Sable ran into the bayou, the police report said.

Two girls were pulled to safety by bystanders as the car sank, but Encalade’s daughters drowned.

The crash prompted some upgrades to Stringer Bridge, where other drivers over the years have driven off the road and into the bayou.

Several civil lawsuits were filed against Encalade, the state Department of Transportation and Development, and Ascension Parish. In September, a judge dismissed the parish and DOTD from the consolidated litigation.

Encalade did not answer a call on Tuesday to her cellphone. Her civil attorney also did not return a call for comment.

Encalade told state troopers that she had taken a Lortab about 5 p.m. the day of the crash, but troopers at the scene initially did not suspect drug or alcohol use as a factor in the crash and believed she went off Stringer Bridge because she stopped paying attention to the road, a police report found.

Encalade told troopers that the children were fighting and “she took the curve too fast and lost control,” the report said.

But a State Police blood test did find later that Encalade had hydrocodone, diazepam, venlafaxine and norvenlafaxine in her system, though no alcohol.

Under state law, drivers are presumed to be intoxicated if their blood-alcohol level is 0.08 percent or greater.

Babin, who in the weeks after the 2012 crash had talked about charging Encalade, said that with drugs, a different burden exists.

“In drugs, there is no such presumption. You have to prove the person was under the influence of an intoxicating narcotic and the scheduled drug impaired your ability to operate a vehicle.”

Babin said that not enough of Encalade’s blood sample remained for his office’s hired toxicology expert to determine how much of the narcotics was in her system.

“I think they needed a lot more sample than they had,” Babin said.

He added that other possible leads that might have pointed to Encalade’s level of intoxication also dried up.

Follow David J. Mitchell on Twitter, @NewsieDave.