WASILLA, Alaska – The mother of Bristol Palin’s fiance was arrested Thursday and charged with six felony counts of misconduct involving a controlled substance.

Sherry L. Johnston, 42, was arrested at her home in Wasilla by Alaska state troopers with a search warrant in an undercover drug investigation.

Johnston is the mother of Levi Johnston, the 18-year-old who received international attention in September when Gov. Sarah Palin and her husband, Todd, announced their teenage daughter was pregnant and he was the father. Bristol Palin, 18, is due on Saturday, according to a recent interview with the governor’s father, Chuck Heath.

Troopers served the warrant at Johnston’s home at the “conclusion of an undercover narcotics investigation,” said a statement issued Thursday by the troopers as part of the normal daily summary of activity around the state.

Troopers charged Johnston with second-degree misconduct involving a controlled substance – generally manufacturing or delivering drugs – as well as fourth-degree misconduct involving controlled substances, or possession.

Troopers released no other information, including the kind or amount of drugs, because details could jeopardize an investigation, spokeswoman Megan Peters said.

Asked how long the investigation had proceeded before Johnston’s arrest, Peters would only say “a while.”

The Palmer District Attorney’s office had no comment.

Sherry Johnston was arrested around noon and booked at Mat-Su Pretrial Facility, according to a booking officer there. She was released on a $5,000 unsecured bond.

No charging documents had been filed at Palmer courthouse by the end of the day, a clerk said.

Levi Johnston sat with Bristol and the rest of the Palin family in St. Paul, Minn., during Gov. Palin’s speech to the Republican National Convention, and he joined the family on the stage afterward.

When asked about the arrest, Palin’s spokesman, Bill McAllister, issued the following statement by e-mail: “This is not a state government matter. Therefore the governor’s communications staff will not be providing comment or scheduling interview opportunities.”

Johnston didn’t come to the door of her home on Thursday afternoon. A teenage boy who answered the door said he couldn’t provide any information.