Filed at 7:59 a.m. ET

WASHINGTON (AP) -- Scientists analyzing anthrax have found tiny differences that might help identify the source of the bacteria used in the fall attacks. With the mailer still at large, the FBI planned to double the reward to $2.5 million.

Three months after anthrax hit Capitol Hill, senators and staff returned Tuesday to the nine-story Senate Hart Office Building, home to half of the 100 senators and closed since an anthrax-laden letter was opened there Oct. 17.

``I go in with confidence and a prayer,'' said Sen. Joseph Lieberman, D-Conn.

In the investigation, scientists hope that identifying genetic markers will allow them to trace the anthrax used in the attacks to one of about a dozen labs that held samples of the commonly held Ames strain.

Until now, no differences among the various anthrax samples had been pinpointed. But scientists at the Institute for Genetic Research in Rockville, Md., now say there appear to be a few subtle genetic variations between two anthrax samples they are comparing: anthrax used in the Florida attack and anthrax held by a British biodefense lab that originally received its sample from the U.S. Army lab at Fort Detrick, Md.

The differences still must be verified, a process now under way by researchers at Northern Arizona University, said Timothy D. Read, who heads the Institute for Genetic Research's effort to map anthrax's genes.

But this could lead to a break in the investigation, Read said Tuesday.

``It has potential,'' he said.

Read added that his research is on the cutting edge of genetic inquiry so scientists are not certain about their findings. ``This type of analysis is going to be something for the future. This is sort of the pioneering effort.''

A law enforcement official confirmed Tuesday that the findings have potential but cautioned that it is unknown whether they will lead to a break in the case.

``We're cautiously optimistic,'' the official said, speaking on condition of anonymity. ``The reality is we're awaiting the results of testing. The reality is we don't know the answer.''

The Bush administration's top bioterrorism official was also skeptical, saying the genetic differences found may be smaller than the margin of error in looking for such differences.

``There's a feeling that (the finding) sounds a little better than it really is,'' said Dr. D.A. Henderson, who directs the bioterrorism office at the Department of Health and Human Services. ``I'm skeptical it's going to be prove to be that definitive, but you never know.''

Investigators still don't know who sent anthrax-tainted letters through the mail that have killed four people and infected more than a dozen others. Most victims were postal workers or people working at media offices where detectives have confirmed or suspect that anthrax letters were sent.

The FBI has made scant progress in its investigation and plans to double the $1.25 million reward in hopes someone will come forward with information.

The new, $2.5 million reward -- including $2 million from the FBI and the U.S. Postal Service and $500,000 from Advo Inc., a major direct mailer -- will be announced at a news conference Wednesday in Trenton, N.J.

The FBI might be able to narrow the field of suspects if Read's discovery is verified and anthrax held by some research labs is found to be genetically more similar to the bacteria used in the Florida attack than anthrax held by other labs. But it is also possible that the anthrax held by various labs is genetically indistinguishable, leaving researchers essentially where they are today.

The anthrax attacks came to light after Robert Stevens, a photo editor for Florida-based tabloid publisher American Media Inc. was infected. Stevens later died.

Anthrax, like other bacteria, grows by replicating itself. Each replication is virtually identical to its source. But sometimes small differences are created in copying the 5 million DNA base pairs that make up the bacteria's genetic structure.

The Institute for Genetic Research, known as TIGR, began its sequencing of the genetic makeup of anthrax two years ago. After the attacks, it got a $200,000 grant from the National Science Foundation to expand its analysis to the anthrax used in Florida.

Its success in finding genetic differences between the two anthrax samples was first reported Tuesday by The New York Times.