McNamara said she did not know why Morrison'sphoto was among those included in the line-up.
"It's possible it got in from anywhere," shesaid.
Citing the ongoing investigation, McNamarawould not say whether Morrison was identified asthe suspect who displayed the knife.
The two other suspects remain at large.
Security has been tightened from the River tothe Quad amid a spree of high-profile crimes.
In four separate crimes since March 19, sixHarvard-affiliated people have been victims ofassault.
Yesterday's arrest is the second piece of goodnews for the HUPD in the past month.
Two weeks ago, investigators presentedinformation to a grand jury about a suspect in theMarch 9 rape of a Harvard-affiliated woman inByerly Hall.
"This is what happens when we follow up all ofour leads," McNamara said of the Mather Housearrest. "We're successful."
Morrison was not available for commentyesterday