Multiple U.S. intelligence and law enforcement officials told NBC News that the Manchester bomber was known to the U.S. intelligence community before the attack.

Those officials also said that Salman Abedi had traveled outside of the UK and had recently returned. As one official put it, "This is a similar situation to the brothers in Boston. We wish we knew more about the travel."

Another official said, "We knew he was going a little crazy and left, but he went quiet and returned a few days ago, and we had no information about him in between."

Multiple U.S. officials with knowledge of his travel say he had previously been in Libya and may also have traveled to Syria.

Related: Manchester Bombing Suspect Was Allegedly Part of a Network

What is not known, according to multiple senior U.S. officials briefed on the UK investigation and officials briefed on U.S. intel efforts, is whether or not Abedi had any training or specific ties to foreign terrorist groups.

Right now, those officials said, they just don't have the hard evidence to tie Abedi to any one group. But law enforcement officials in the UK and the U.S. are working their sources to try to find out what he was up to.