A federal grand jury in Boston is investigating Millennium Laboratories of San Diego, a fast-growing private company selling urine drug testing services to pain clinics across the United States.

The company not only is under investigation by the Justice Department for allegations of health care fraud but also for intimidating former employees, one who was portrayed in a slideshow at a company meeting as a corpse in a body bag.

Two of the ex-employees, who had raised concerns about Millennium’s sales practices, also say they were followed for weeks by private investigators they believe were hired by the company.

No criminal charges have been filed, and Howard Appel, Millennium’s president, said the company is cooperating fully with a Justice Department subpoena and did nothing wrong.

Appel said the company is a leader in business ethics in the estimated $4 billion industry that helps doctors monitor the soaring use and abuse of pain drugs. He said the grand jury may also be investigating Millennium’s competitors.

Four witnesses, speaking publicly for the first time, described their grand jury testimony to Reuters in separate interviews. They said they were only asked about Millennium.

Reuters reviewed copies of five grand jury subpoenas seeking records on Millennium. Federal grand juries operate in secrecy to investigate matters that might constitute criminal conduct. Witnesses are free to describe what they said.

All four said they testified that Millennium was getting doctors to order unnecessary urine tests and charging excessive fees to Medicare and private insurers. Millennium has denied those accusations in civil lawsuits.