The AFL anti-doping tribunal has acquitted 34 current and former Essendon footballers of injecting a banned substance.

On Tuesday afternoon, the tribunal delivered the surprise verdict, which is a blow to ASADA’s credibility, clearing the players to compete in round one.

The tribunal found unanimously it was “not comfortably satisfied” any player was given the banned synthetic substance thymosin beta-4.

• Not guilty: Essendon players cleared
• Steak theory rejected in AFL doping case
• CEO calls for AFL drug review

ASADA has issued a defiant statement and has not ruled out an appeal.

“What happened at Essendon in 2012 was, in my opinion, absolutely and utterly disgraceful.  It was not a supplements programme but an injection regime and the players and the fans were so poorly let down by the club,” said ASADA chief executive Ben McDevitt.

“While I am obviously disappointed that the charges in this instance have not been proven to the comfortable satisfaction of the tribunal, I am pleased that the tribunal was able to finally hear these matters,” he said.

The league itself has 21 days to consider an appeal, the anti-doping rules state.

ASADA and the World Anti-Doping Agency (WADA) may also launch an appeal within 42 days.

It was each player’s duty under the code to ensure nothing illegal entered their bodies, so they could not argue ignorance.

Thus, the verdict related exclusively to a lack of proof that sport scientist Stephen Dank injected them with a banned substance.

Mr Dank gave sworn evidence to the Australian Crime Commission (ACC) that he gave the players the immunity booster thymomodulin, not thymosin beta-4.

Both are types of peptides, but only thymosin beta-4 is illegal for use by athletes.

Thymomodulin is a collection of peptides extracted directly from the thymus gland of cows, whereas thymosin beta-4 is one particular type of cow-sourced peptide synthesised in a lab.

It is theoretically possible that thymomodulin could contain the natural form of the synthetic peptide that is thymosin beta-4.

Several Essendon players reportedly told ASADA they were injected with something called “thymosin” — a word that can refer to both the legal and illegal peptide.

Essendon captain Jobe Watson reportedly told ASADA he was injected with the legal peptide, not the illegal form.