A member of Health Minister Jane Philpott's staff has lost his job after he gained unauthorized access to NDP offices near Parliament Hill.

Mark Livingstone inappropriately entered the New Democrats' research office on Sparks Street around 9:30 p.m. on Nov. 3, 2016, the day before the Liberal government's one-year anniversary in power. On Nov. 8, the Liberal House Leader's chief of staff, Rheal Lewis, notified NDP Whip Marjolaine Boutin-Sweet a Liberal staffer had entered the office days earlier.

According to NDP deputy chief of staff Lucy Watson, the party learned who the staffer was only on Tuesday – the day The Globe and Mail asked Ms. Philpott's office about the incident – in a letter Mr. Livingstone dropped off at the office for NDP Leader Tom Mulcair.

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"On November 3, 2016, I was enjoying an evening of celebrations at an establishment located in the basement of 202 Sparks Street," Mr. Livingstone's letter said.

The Cock and Lion, a pub frequented by Liberal staffers, is below the NDP research bureau. Mr. Livingstone worked in that office when it was the Liberal research bureau during the party's time in opposition.

"Out of nostalgia for my old office, I briefly entered the common area of the office at 202 Sparks Street," the letter continued. "I acted alone and there was no malicious intent behind my sentimental actions. I regret this lapse in personal judgment and apologize for my actions."

Ms. Watson said Mr. Mulcair is deeply concerned.

"[He was] surprised, dismayed that our repeated efforts [to get information about the incident] had not been met with any sort of co-operation, that there were so many questions that we had not received answers to."

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Mr. Livingstone could not be reached for comment, and the Liberals would say only that he does not work for the government any more.

The NDP said it was told Mr. Livingstone did nothing while he was briefly in the office, where senior staff, media relations and research teams work. However, the New Democrats want to know how Mr. Livingstone got past security, given that he had no pass for the building.

"We don't know how long this individual was in the office," Ms. Watson said.

"We don't know if this person was alone or with a group. We don't know whether he removed documents or whether they removed documents. And we also don't know why we got a letter yesterday claiming responsibility and claiming he acted alone."

The New Democrats said they attempted to follow up with the Liberal House Leader's office after they were notified and were directed to Parliamentary security, which told them it had footage from cameras at the main entrance of the building and the second-floor reception area in the NDP's office.

However, a few days later, the NDP said the House of Commons corporate security operations chief, Michel Denault, told them they could not see the video. The party said that when asked for a summary of the tape, Mr. Denault responded that the footage was not available because of a "programming oversight" and refused to identify the intruder.

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On Nov. 15, the NDP learned that a note was left for a security guard the night of the intrusion that suggested more than one person was involved. Security for 202 Sparks Street is contracted out to a third-party provider.

"We miss you and hope you are well!" read the note, which was signed "Your former tennants."

Heather Bradley, director of communications for the House of Commons Speaker, said in an e-mail that the office of the Sergeant At Arms took steps to prevent similar incidents.

Ms. Watson said staff must now check in with a security guard when entering after business hours.