No, i dont think we know that. i will try to find the article where the Imam, or perhaps just a spokesperson at the cultural center in Cambridge said he had only visited once.
Attendance at an American mosque is not a prerecquisate to having radical Islamist views. We know that from the 9-11 terrorists. You don't have to be carrying a KKK membership and be at the weekly Skinhead meeting to hold racist views...and if there are expressions and links to hate....would you say "we can't know that" ...if this were a Skinhead who terrorized a Black church.
I don't believe thatAmerican mosques teach Jihad...so the attendance or non attendance of these brothers is meaningless. They were enamoured by Jihad.
We have seen countless articles where family, friends, politicans on all sides, FBI, etc ..even the government of Russia understood that the older brother had become strident in his religion and his Jihadist tendencies. We have seen evidence that he had great influence on his younger brother.
I think there is only a miniscule chance...if even that...that we do not know the part radical Islam beliefs by these brothers played in this atrocity.
Even one American Iman today said he would not afford the older one a Muslim burial. That Iman understands that radical jihadists are an abomination to his Faith...so no need to pretend these brothers were just fascinated by pressure cookers.
Jihadists are not devout Muslims...they just think they are...to the horror of humanity.
http://religion.blogs.cnn.com/2013/04/21/imam-i-wouldnt-give-boston-suspect-last-rites/?hpt=hp_t2
A least one Boston cleric said he would refuse to perform funeral rites for a man accused of committing so much violence. The Quran, said Imam Talal Eid, says that anyone who has killed another human being is going to hell.
Eid, who is imam at the Boston Islamic Institute, said he had never met the Tsarnaev brothers but questioned media accounts that Tamerlan Tsarnaev had become a devout Muslim.
"A person who is devoted does not kill innocent people," Eid said.
Yusufi Vali, executive director of the Islamic Society of Boston Cultural Center, the largest mosque in the Boston area, also rejected the Tsarnaev brothers.
"I don't care who or what these criminals claim to be, but I can never recognize these criminals as part of my city or my faith community," he said.