An Arlington County police investigation into whether a taxpayer's check to the county treasurer's office was altered has ended inconclusively.

"We don't know who altered the check," which was allegedly transformed from one for $26.22 into one for $262.20, said county prosecutor William S. Burroughs Jr.

He said he has "been advised by the police that there is nothing more that can be done" to determine who might have changed the figures on the personal property tax check written last summer by Karen Troutman.

Troutman, a government secretary in her 20s, has said she was unaware that she had underpaid taxes on her car until she got a notice from her bank saying her checking account was overdrawn.

Her husband, Robert, said the couple was puzzled and, after going through canceled checks, found the numbers on the original check had been changed and the correct higher amount filled in.

In an interview last night, county treasurer Bennie L. Fletcher said his office "never alters a check," and "didn't alter this one." Office rules "are that nobody in the office alters a check," Fletcher said.

The treasurer said he has been conducting his own investigation into who altered the check. Although it, too, has been inconclusive, Fletcher said, a handwriting expert he called in "concluded that nobody in the treasurer's office did it, anyway."

As part of the police investigation, several lie detector tests were administered to persons in his office, Fletcher said, and they "all came out clean."

Burroughs said the police "have no further leads to follow . . . and therefore the investigation is closed."