For nearly two centuries, scholars have debated whether some 325 lines in the 1602 quarto edition of Thomas Kyd’s play “The Spanish Tragedy” were, in fact, written by Shakespeare.

Last year, the British scholar Brian Vickers used computer analysis to argue that the so-called Additional Passages were by Shakespeare, a claim hailed by some as the latest triumph of high-tech Elizabethan text mining.

But now, a professor at the University of Texas says he has found something closer to definitive proof using a more old-fashioned method: analyzing Shakespeare’s messy handwriting.

In a terse four-page paper, to be published in the September issue of the journal Notes and Queries, Douglas Bruster argues that various idiosyncratic features of the Additional Passages — including some awkward lines that have struck some doubters as distinctly sub-Shakespearean — may be explained as print shop misreadings of Shakespeare’s penmanship.

“What we’ve got here isn’t bad writing, but bad handwriting,” Mr. Bruster said in a telephone interview.

Claiming Shakespeare authorship can be a perilous endeavor. In 1996, Donald Foster, a pioneer in computer-driven textual analysis, drew front-page headlines with his assertion that Shakespeare was the author of an obscure Elizabethan poem called “A Funeral Elegy,” only to quietly retract his argument six years later after analyses by Mr. Vickers and others linked it to a different author.

This time, editors of some prestigious scholarly editions are betting that Mr. Bruster’s cautiously methodical arguments, piled on top of previous work by Mr. Vickers and others, will make the attribution stick.

“We don’t have any absolute proof, but this is as close as you can get,” said Eric Rasmussen, a professor at the University of Nevada, Reno, and an editor, with Jonathan Bate, of the Royal Shakespeare Company’s edition of the complete Shakespeare.

“I think we can now say with some authority that, yes, this is Shakespeare,” Mr. Rasmussen said. “It has his fingerprints all over it.”