It was shown vs it was showed

Some examples from the web:

• It was shown in feeding experiments that the oxidative stress (lipid peroxidation) in diabetic rats was reduced by chlorella treatment.
• It was shown that only 1 % of the iron oxides and hydrated iron oxides were likely to become solubilised in the human gastrointestinal tract and therefore the Committee established an acceptable daily intake without specifying an upper limit.
• The inauguration of "Kcho Study Romerillo, Laboratory for art"was in January last year when Fidel Castro attended (last time it was shown publicly).
• 8 You shall not make it solid, but empty and hollow at the interior, just as it was shown to you on the mountain.
• It was shown that love is stronger than death.
• The premises were not the best. 14 hours to reach their destination, but then opened the gate, it was shown to us an enchanted world, made of perfumes, colors, exquisite delicacies prepared with kindness and wisdom and skill.
• It was shown in a clinical study that concomitant use of ropinirole with ciprofloxacin, a moderate inhibitor of the CYP450 1A2 isozyme, results in an increase of Cmax and AUC of ropinirole by 60% and 84%, respectively.
• Indeed, in a number of cases it was shown that the imported products from the PRC and Thailand were sold via the same distributors.

Some examples from the web:

• It was showed in Florence the establishment project of Italia Covention Bureau, the new institution designated to coordinate the promotion of Italy as headquarters of event of any type towards abroad.
• [...] Some time ago it was showed off in Locri a statue of the cithara
• bard Eunomus with a cicada seated on the cithara. Timaeus affirms that Eunomus and Ariston of Rhegium were once contesting with each other at the Pythian games to gain the victory.
• And I remember thinking how strange it was That it showed up in all three of the men.
• The endoscope showed it was esophagus cancer.
• It was until you showed up, loco.
• It was you who showed me.
• Actually, it was Chuck who showed great courage on this mission.