New findings have raised concerns about fresh anthrax attacks in the United States.

Investigators have revealed that the spores used in mail attacks last year, killing five people, were less than two years old.

"It's modern," a government official told the New York Times.

"It was grown and therefore it can be grown again and again."

Male loner

The FBI used radiocarbon dating to establish the age of the anthrax, which had been sent to the US Congress and prominent media figures.

The claim gives new credence to suspicions that the attacker has access to a microbiology lab.

It was first suspected that Osama Bin Laden's al-Qaeda network was behind the attacks.

But now suspicions point towards a male loner with a scientific background and a grudge against society.

Frustration is growing in the US at the failure by authorities to arrest the culprit.

Californian Democratic Senator Barbara Boxer told Congress that too much time was being spent on creating an anti-terror bureaucracy, and not enough on finding the killer.

$2.5m reward

She said: "That anthrax killer is out there. We need to nab this person."

However, investigators say their failure to crack the case is not through want of trying.

Agents have conducted 5,000 interviews and served more than 1,700 grand jury subpoenas - all at the cost of millions of dollars, but to no avail. So far, even the offer of a $2.5m reward has failed to produce a breakthrough.

All mail addressed to the US Government is now irradiated to kill anthrax.

The FBI is also giving lie detector tests to workers at a Utah facility where the bacteria are stored.

And while authorities acknowledge that they have no prime suspect in the case, investigators have a list of about 50 people, according to the New York Times.