The slasher may have had his weapon concealed beneath a shopping bag as he crept up behind a pregnant woman at Fairfax County’s Fair Oaks Mall, poised for an attack, a prosecutor said.

The woman, 20, noticed the man hovering behind her and then felt a sharp pain. Before she determined what had happened, he slipped off into the crowd.

“She looked at her rear end and saw her pants were cut,” Assistant Commonwealth’s Attorney George Freeman told a judge. “She had a cut on her buttocks that was bleeding.”

Johnny D. Guillen Pimentel, 42, a native of Peru, admitted his guilt Monday in the February 2011 slashing and others at Northern Virginia shopping centers as part of a plea deal with county prosecutors in a case so bizarre it grabbed headlines locally and nationally.

Guillen Pimentel was suspected in nine slashings at Fairfax malls in 2011, but under the terms of the deal reached Monday he pleaded guilty to two counts each of malicious wounding and unlawful wounding in four of the attacks. The deal calls for a maximum sentence of seven years in prison.

Johnny D. Guillen Pimentel (Courtesy of Fairfax County Police)

In each case, Guillen Pimentel would sneak up behind women in their teens or 20s and slash their buttocks using a hobby knife before disappearing into the busy shopping centers.

None of the attacks caused serious injuries, but the Fair Oaks Mall victim required medical attention and she was left with a permanent scar, Freeman said.

The attacks at Fair Oaks, Tysons Corner Center and the Greenbriar shopping center in Fairfax unnerved customers and led police to form a task force to catch the culprit.

Although the case is winding down, one of its central mysteries remains: Why did the onetime day laborer do it? Freeman did not give a motive and the defendant did not have the opportunity to address his crimes in Fairfax County Circuit Court.

Wearing a green prison jumpsuit and speaking through a Spanish interpreter, Guillen Pimentel gave yes-or-no answers to a judge’s questions.

After the slashings, Guillen Pimentel fled to Peru, where he was arrested in Lima in January 2012 and brought back to the United States. The case drew a flurry of coverage there — some media outlets used actors to portray the attacks.

A member of Guillen Pimentel’s family declined to comment on the case in court. Guillen Pimentel is scheduled to be sentenced Sept. 6.