INDIANAPOLIS - Investigators have busted a child pornography ring spread across the United States and Europe that produced and distributed sexually explicit images of babies and toddlers online, federal prosecutors in Indianapolis said Thursday.

Seven American men have been convicted and sentenced on various charges in the case, including three who were sentenced in federal court in Indianapolis on Wednesday, the US attorney’s office said. Two more who pleaded guilty are awaiting sentencing.

“This operation uncovered a dangerous and depraved group of criminals who were devoted to trading sexually explicit images of children under the age of five,’’ Assistant US Attorney General Larry Breuer said in a statement.

More than 20 suspects have been captured in nine states, and authorities are investigating yet more elsewhere in the United States, as well as Sweden, Serbia, the Netherlands, and the United Kingdom.

First Assistant US Attorney Josh Minkler said at an Indianapolis news conference that more than two dozen children in Indiana and elsewhere were abused in the production of the pornography.

“Children who far too often weren’t old enough to comprehend the crimes committed against them,’’ Minkler said.

Some of the suspects had legal custody of their victims, and those children have been removed, prosecutors said.

The “most prolific producer of child pornography’’ in the group was 26-year-old David Bostic, Assistant US Attorney Brant Cook said.

Bostic persuaded parents to allow him to baby-sit and, thus entrusted, abused the children without their knowledge, Assistant US Attorney Steve DeBrota said. He was arrested in November 2010.

He was convicted on 65 counts and sentenced to 315 years in prison. Bostic’s arrest heralded the launch of Operation Bulldog; further investigation of his computer led investigators to others.