Twenty years ago, on 19 April 1995, a disaffected veteran named Timothy McVeigh drove a Ryder truck stuffed with explosives into downtown Oklahoma City and destroyed a federal office building, killing 168 people, including 19 children, and maiming hundreds of others. That much we know.

We also know that, within 90 minutes of the bombing, McVeigh was pulled over near the Kansas border and arrested, alone, at the wheel of a glaringly improbable getaway car, an ancient, spluttering rust bucket of a Mercury sedan with no licence plates which made him a sitting duck for any passing highway patrolman.

How could such a callous, carefully planned attack have come to such an incongruously slapdash end? After a vast investigation headed by the FBI, three trials mounted against McVeigh and his co-conspirator, Terry Nichols, and an avalanche of court documents, there is still no definitive answer to that question.

Perhaps the most striking thing about the Oklahoma City bombing – by far the most destructive act perpetrated by a home-grown assailant against fellow Americans – is not how much we've learned over the past 20 years but rather how much we still do not know.

Despite the government's insistence that the case has been solved, we don't know the exact origin of the plot or how many people carried it out. The federal indictment against McVeigh and Nichols – the latter fronted the money and did most of the bomb's construction for McVeigh – made specific mention of "others unknown", and when their trials were almost over, the presiding judge publicly urged the FBI and other law enforcement agencies to keep investigating. The plea fell largely on deaf ears.

We don't know how McVeigh and Nichols learned to build a fertiliser bomb of such size and power. (Neither received more than rudimentary explosives training when they served together in the Army, and their early experiments with smaller devices were haphazard at best.) We don't know the identities of the other people seen with McVeigh on the morning of the bombing – only that more than 20 eyewitnesses were unanimous in telling the FBI he was not alone.

There is no ready explanation for a different Ryder truck seen by witnesses at McVeigh's motel in Kansas and at the state park where the bomb was assembled in the week leading up to the bombing; no explanation for the other people seen inside McVeigh's motel room during the same period; no satisfactory explanation of the fact that two people were seen renting the bomb truck on 17 April, neither of them entirely fitting McVeigh's description.

An examination of the official investigative files on the Oklahoma City bombing – about a million pages of material – does little to bolster the assertion of Frank Keating, a former FBI agent who was Oklahoma governor in 1995, that "two evil men did this and two evil men paid".

Rather, it does the opposite. The impression, confirmed by the memories of front-line investigators and lawyers who prosecuted the case but did not speak independently about it for many years, is of leads left dangling or shut down instead of being pursued with the FBI's customary vigour. Obvious suspects were offered deals by government prosecutors, usually but not always in exchange for their testimony. Others slithered down the priority list until they were lost or forgotten. Half a dozen rightwing radicals fingered as possible suspects by government informants or by fellow anti-government warriors were not questioned about the bombing, even when it became clear they had lied about their whereabouts on 19 April.