Investigators probing the anthrax-by-mail attacks in the United States have doubled the reward they are offering for information on the case.

Federal investigators have increased the reward for information on the anthrax attacks to $2.5 million, up from the one-point two million dollars announced late last year.

They are also circulating fliers that show pictures of the four envelopes that contained anthrax to half a million postal customers in central New Jersey and neighboring Pennsylvania in hopes of stirring up new leads in the case.

Investigators believe the letters containing anthrax were mailed from the Trenton, New Jersey, area and they say they will continue to focus their efforts there.

Postal inspector Kevin Burke told reporters Wednesday that investigators have only a rough idea of who they are looking for.

"We believe the persons responsible for these acts obviously have a scientific background and work history, which may include a specific familiarity with anthrax and have a level of comfort in and around the Trenton, New Jersey, area due to present or prior association," he said.

FBI agent Kevin Donovan says the federal anthrax probe remains aggressive and wide ranging. But he also says that investigators could use some help from the public.

"I don't think it is that we don't have any leads. We are looking out for that one person who may be able to give us specific information to help us utilize the investigation that has already been conducted and the scientific information that the laboratories around the United States are trying to give us so we can focus on that information to identify one particular individual," he said.

The anthrax attacks came in the wake of the September 11 terrorist attacks and targeted two U.S. Senators and national media outlets. Five people died after inhaling anthrax spores and several others received medical treatment as a result of their exposure.

In addition, federal officials have recorded some 15,000 anthrax hoaxes and arrested 71 people in the process.

Federal investigators have not ruled out a connection between the anthrax attacks and the September 11 terrorist attacks. But several law enforcement experts believe it is more likely that the anthrax mailings were the product of a homegrown terrorist or disturbed individual with some sort of a grudge against the government or the news media.