Fentanyl, a powerful synthetic opioid implicated in nearly 29,000 overdose deaths in the United States last year, most likely spread because of heroin and prescription pill shortages, and also because it was cheaper for drug wholesalers than heroin, according to a report on illicit US drug markets by researchers at UC San Francisco.

First synthesized in a Belgian lab in 1960, illicitly produced fentanyl has swept through U.S. drug markets in waves, the latest beginning in 2013, and has claimed an extraordinary number of lives. The researchers who wrote the report said having a better understanding of whether fentanyl's deadly spread is being driven by the economic imperatives of suppliers or the tastes of drug users would help policymakers find ways to curb it.

Made entirely in the lab, fentanylSchools Lack Full Slate of Teachers

Loudoun County Public Schools needs more special education teachers.

Despite year-round recruiting efforts, Loudoun County Public Schools is expected to start the fall without a full slate of teachers for the academic year.

Loudoun County Public Schools has been hiring about 500 new teachers each year since fiscal year 2002. They needed 630 additional teachers this fall. On Monday, they had hired 586, and were still looking for 44. They also were trying to fill about 74 other school vacancies, such as secretaries, cafeteria workers, teacher assistants and custodians.

"At one point, there was a surplus of applicants so we could pick and choose who we wanted," said Matthew D. Britt IV, assistant superintendent for Personnel Services. "Now it is reverse. We have to beat the