At least one adult altered the answer sheets on standardized tests at a Las Vegas elementary school, leading to skyrocketing scores from one year to the next, an investigation by the Nevada Department of Education concluded.

The department issued a statement Wednesday saying the 2011-2012 test scores at Kelly Elementary School will be invalidated, and letters of admonition concerning the principal and assistant principal will be forwarded to the Clark County Board of Education for possible disciplinary action.

"I have no doubt that a testing irregularity occurred at this school, and that student answer sheets were altered by one or more adults in the system," said state Superintendent Dale Erquiaga. "Testing security procedures were also breached. However, the investigation has not yielded the identity of the individual or group of individuals who changed the answers in 2012."

School administrators were not available to comment on the development. A receptionist at the school said Wednesday they were on spring break.

The investigation into Kelly Elementary's suspicious scores began after a tipster submitted an anonymous report in April 2012, alleging that the school's principal coached students to change the answers on a statewide Criterion Referenced Test administered in the spring of 2012. An associate superintendent interviewed the principal in May, asked her whether she had coached the students, and then concluded the investigation when the principal said she hadn't, the state report said.

But scores released in June 2012 showed eyebrow-raising improvement at the low-performing school. State data show that from 2008 to 2011, the number of Kelly Elementary fifth-graders considered proficient in reading never reached 25 percent. But that figure leaped to nearly 78 percent for the 2011-2012 school year, and hovered at 72 percent in 2012-2013.

The results fit neatly into the narrative of reforms transforming a struggling school, the report said: a low-achieving school implemented major changes, test scores turned around and student success was celebrated. When the scores were released, school officials provided no explanation for the dramatic improvement aside from saying students tried hard and reforms were in place, the report continued.

During the investigation, which also included the Nevada Attorney General's office, an erasure detection analysis found a disproportionate number of answers were changed from wrong to right.

When the district supervised the test in 2013, scores declined significantly, education officials said.

The education department deposed or interviewed fifteen witnesses, including teachers who gave the test, the school's office manager and administrators. Investigators also reviewed more than 2,000 pages of documents obtained through subpoenas, officials said.

Clark County School District Superintendent Pat Skorkowsky said he's grateful for the investigation and will review the findings to determine next steps.

"It is important that our community have faith in the validity of our standardized testing," he said in a statement. "Once I have had a chance to review the department's full report and recommendations, I will take appropriate action so our students, parents and community can move forward."