質問 #25 friends with~

このところ日本ではバラバラ死体の事件が相次いでいます。英国でも猟奇的な臭いのプンプンする事件が話題となっています。UGが習慣としている「新聞電子版拾い読み」から。
Crime student questioned over murder of three prostitutes
A crime student who specialised in researching serial killers is being linked to the murders of up to six women on the streets once stalked by the Yorkshire Ripper.
 
By Paul Stokes, Richard Edwards and John Bingham
Published: 10:21PM BST 26 May 2010
あのJack the Ripper（切り裂きジャック）を彷彿させる事件の容疑者はなんと，その世紀の殺人鬼を研究している（specialise in cf. （米）specialize in）Bradford大学の大学院生（psotgraduate student cf. （米）graduate student ）とのことで
わけのわからない事件です。leadの"six women on the streets"の意味するところは読みとれますね。詳細は；
http://www.telegraph.co.uk/news/uknews/crime/7768939/Crime-student-questioned-over-murder-of-three-prostitutes.html
にあります。
こんな事件を題材にするのは気が引けますが，記事中の；
Separate searches for all the missing women had been continuing throughout Bradford but until yesterday police refused to link them.
Miss Rushworth was the first to go missing, on June 22 last year. She spent the previous evening with her family in Thornton before returning to her bedsit in Manningham.
Miss Armitage was last seen on CCTV cameras at 10pm on Monday, April 26, close to the city centre and again in the red light district.
Miss Blamires went missing on Friday. She was friends with Miss Armitage and lived a few streets away from her in Bradford’s Allerton district.
Mr Griffiths was arrested on Monday and the scope of the inquiry was expanded to link the other women.
の太字・イタリックにした箇所の表現，何か気づきませんか。a friendと数えたいところですが，friends with〜と複数となっています。なぜかな。誰か助けてください。（UG）