Their aim was to test whether pollen from the maize could affect insects that live in nearby plants.
Researchers in the US say they have found more evidence that pollen from maize genetically engineered to resist a common pest could kill Monarch butterflies.
The finding of the two-year study, carried out in small-scale maize plantations in the state of Iowa, adds weight to a similar study published last year.
The original research found that maize plants that had been genetically engineered to have insecticidal properties against the corn-borer pest also killed Monarch butterfly larvae in the laboratory.
The US Environmental Protection Agency says it is reviewing the new data.
Summer migration
Monarch butterflies pass through the so-called "cornbelt" in the mid-western US on the way to and from their winter sanctuary in Mexico.
Researchers at Iowa State University placed pots of milkweed plants - a favourite food of Monarch butterflies and a common site for laying their eggs - around a field of the insecticidal maize.
Their aim was to test whether pollen from the maize could affect insects that live in nearby plants.
They found that Monarch butterfly caterpillars were seven times more likely to die from eating milkweed near the genetically-modified (GM) maize compared with those that ate milkweed leaves with no GM pollen on them.
But the company that makes the GM crop, Novartis, says its maize is safe, and says the study did not duplicate real-world conditions.
'Buffer zone'
The environmental group Greenpeace has called for a "buffer zone" around GM maize fields.
The US Environmental Protection Agency (EPA) says it will review the Iowa study, and make a statement early next year.
But an EPA spokesman, Steve Johnson, told reporters that other studies had not shown the maize had a detrimental effect on nearby insects.
The new data is published in the journal Oecologia.