When an anthrax investigation at Fort Detrick, Md., wrapped up last month, FBI agents carried out at least three coolers from the building where the dangerous bacteria is stored, a source told FOX News.

Investigators with the "Amerithrax" task force, which comprises FBI and postal service agents, took their three-years-long hunt to the U.S. Army Medical Research Institute of Infectious Diseases (search) (USAMRIID) at Fort Detrick, Md., earlier this summer. The Fort Detrick portion ended July 23.

Agents also searched computers and discs, and some equipment may have been removed from USAMRIID, sources said.

It has long been speculated that the anthrax (search) in tainted mailings that killed five people and sickened 17 shortly after the Sept. 11 attacks was somehow stolen from the Fort Detrick labs.

In a separate development, suggesting the government's investigation continues to move forward, FOX News has learned that at least four scientists affiliated with Fort Detrick (search) were asked to testify before a grand jury, in Washington, earlier this summer.

Law enforcement officials are unwilling to discuss this matter given the secret nature of the grand jury. A Fort Detrick spokesman would not comment directly, saying only that scientists have been fully cooperating with the investigation.

In addition, FOX News has learned some scientists at USAMRIID were questioned about the data in their lab notebooks, which include the toxic agents, procedures and personnel present for all experiments.

It is widely believed that the anthrax used in the deadly letters sent in September and October 2001 was stolen from the Army's premier bio-weapons lab. Neither Army nor FBI officials are willing to confirm this on the record. 

The site is also significant because of its connection to Steven Hatfill (search), a biochemist who once worked at the labs. U.S. Attorney General John Ashcroft has described him as a person of interest in the case. Hatfill denies the charges and is now suing the government.

Also, Kenneth Berry, an emergency room physician and the founder of an anti-terrorism organization that trains medical professionals to respond to chemical and biological attacks, recently became the focus of FBI searches in the probe.

Berry reportedly presented a paper at Fort Detrick in 1997. He has denied any connection to the attacks.

The FBI considers the anthrax investigation its most complex ever. About a month after the Sept. 11 attacks, letters laced with the deadly bacteria were mailed to government and media offices, including to Sens. Tom Daschle, D-S.D., and Patrick Leahy, D-Vt., and to The New York Post and NBC anchor Tom Brokaw.

The mailings rattled an already shaken populace, and postal and government buildings experienced closures. No one has yet been charged in the attacks.

FOX News' Catherine Herridge and Anna Stolley contributed to this report.