One man will appear at a Christchurch court on Saturday. She described it as "one of New Zealand's darkest days".
The shootings took place in Christchurch, New Zealand near Cathedral Square and Hagley Park.
Chicago police said there are no known threats to Chicago, but they will be keeping a closer eye on mosques.
He wrote that "49 innocent people have so senselessly died, with so many more seriously injured".
Although police have not revealed the identities of those detained, they did indicate that the names of the suspects are not on any watch list.
In the document, he explicitly stated who he was and his justification for the shooting. Media reports identified the 28-year-old man as Brenton Tarrant.
Australian Prime Minister Scott Morrison has described the gunman, who had Australian citizenship as "extremist, right-wing" terrorist.
The Bangladesh cricket team narrowly avoided being caught up in the shooting while on a bus approaching one of the mosques while in the city for a match, which has since been canceled. He had previously donated to and collaborated with nationalist organizations, though he claims to have acted alone in ordering the attack. It is the second most-populous city in New Zealand behind Auckland and Wellington.
Bishop of Peterborough Donald Allister said: "I was horrified to hear of the shootings at the mosques in Christchurch, New Zealand".
Mayor of London Sadiq Khan also shared a video on Twitter, recording a message that ended: "When the flames of hatred are fanned, when people are demonised due to their faith, when we play on people's fears rather than addressing them, the consequences are deadly, as we have seen so sadly today".
The Pittsburgh synagogue attack prompted an outpouring of interfaith solidarity between Muslims and Jews at the time, with the SAJBD statement - one of several from Jewish organisations so far - showing that the Queensland mosque attack has inspired the same.
Peneha, who lives next door, said the gunman ran out of the mosque, dropped what appeared to be a semi-automatic weapon in his driveway and fled.
He was quoted by 9 News Australia as saying, "I saw dead people everywhere". All Christchurch schools have been shut.
Witness Len Peneha said he saw a man dressed in black and wearing a helmet with some kind of device on top enter the Al Noor mosque and then heard dozens of shots, followed by people running out in terror.
He added: "I am just a regular White man, from a regular family".
Armed police on Linwood Ave and specialist armed police after the Linwood Mosque came under attack.