Since the terrorist attacks of September 11, 2001, senior government officials have announced dozens of terrorism cases that on closer examination seemed to diminish as legitimate threats. The accumulating evidence against a Denver airport shuttle driver suggests he may be different, with some investigators calling his case the most serious in years.

Documents filed in Brooklyn against Najibullah Zazi contend he bought chemicals needed to build a bomb — hydrogen peroxide, acetone and hydrochloric acid — and in doing so, Zazi took a critical step made by few other terrorism suspects. If government allegations are to be believed, Zazi, a legal immigrant from Afghanistan, had carefully prepared for a terrorist attack. He attended a Qaeda training camp in Pakistan, received training in explosives and stored in his laptop nine pages of instructions for making bombs from the same kind of chemicals he had bought.

While many important facts remain unknown, those allegations alone would distinguish Zazi from nearly all the other defendants in US terrorism cases in recent years.

In two cases unrelated to Zazi in which charges were announced on Thursday, in fact, the subjects dealt extensively with undercover agents.

The Zazi case “actually looks like the case the government kept claiming it had but never did”, said Karen J Greenberg, executive director of the Center on Law and Security at New York University law school.

Her centre has studied all the prosecutions of terrorism-related crimes since 2001.

This time, she said, “the ingredients here are quite scary”, and the government’s statements have had none of the bombast and exaggeration that accompanied some previous arrest announcements.

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