News outlets have just reported on this, but not much info was given. I shall just post whatever is on Twitter and come back to update ater.
According to fan accounts, a man holding a sharp object went on a rampage during a AKB48 handshake session today. He seemed to to be queueing in Takamina’s lane Kawaei’s lane before starting to attack. Kawaei Rina (Update 3: Iriyama Anna too) and a staff are reported to be injured. Blood was seen flowing from Kawaei’s head. Screams were heard all around. Police and ambulance arrived at the scene and Kawaei has been sent to the hospital. The extent of her injuries is unknown. Someone said Takamina was shivering in fright. The handshake session has been cancelled for today. Man has been arrested and is currently being investigated for attempted murder.
One picture spreading on Twitter.
Update 1: NHK is saying that there are 3 people injured in total. They did not clarify on the identity of these 3.
Update 2: First person account from a fan who attended the handshake event. Link
Update 3: Yahoo reported that Iriyama Anna was injured as well. Both girls are conscious and not in critical condition. The guy is a 24 year old unemployed from Aomori province.He was holding something that looks like a 50cm saw and was queueing for a handshake when he suddenly lunged at the members.Kawaei was injured in the head while Iriyama was cut in the wrist. Both girls were attending to fans in the same lane, together with members Takajo Aki, Oshima Ryoka and Kuramochi Asuka. Attending the event on the same day were Watanabe Mayu, Kashiwagi Yuki, Kojima Haruna, Takahashi Minami and Shimazaki Haruka. Source
Update 4: No security check on personal belongings was conducted for fans attending this event. Source
Update 5: Seems like the Yahoo report on Kawaei’s head injury was untrue. AKB48 Theater Manager Yuasa Hiroshi revealed that Kawaei and Iriyama have fractured fingers and cuts on their right hand. Iriyama has cuts on her head as well. They have undergone a 3hr surgery at 9pm yesterday to stitch the cuts. Despite the shock, both girls were worried about the handshake event. Source Video Video
Update 6: The attacker’s mother isn’t sure if her son is an AKB fan as their CDs/handshake tickets have never been seen around the house. He had become unreachable after leaving the house on 24 May. Source
Update 7: The girls were discharged from hospital as of 6.30pm on 26th May. They reassured reporters that they are alright. Their hands were covered with a towel. Statements from the attacker has also been revealed. He was looking to kill anyone in a crowded place and has not shown any signs of remorse. Video
Update 8: The attacker did not even know Kawaei and Iriyama’s names. He brought the saw from home and hid it in his personal belongings(some mentioned he hid it in his jumper) while queuing at the event. Source
AllThingsJpop 
why did Yahoo write the name of popular members ??? just to make the news bigger ……… well this is a horrible news AKB made this kind of event practically every weekend, now members are gonna be afraid every moment a fan is near to them, not to mention Kawaei and Iriyama.
I think they wanted to suggest that Mayu, Yukirin etc could very well be the victims too. It’s definitely scary to think what else could have happened. But now i think they are going to step up security…like metal detectors.
I hope not only Japan but other countries will always check fans personal belongings. How in the world did he hide a 50cm/19inch saw??? This could have been avoided had security checked everyone. If his target was a big audience, he could have started while in line before he even got to any of the members. Thank goodness he didn’t have a gun.