A small fire broke out in the women’s restroom on the second floor of the Dirksen Senate Office Building on Friday morning, the second in three days and the seventh in a month.

It’s the latest installment in the mystery surrounding a series of fires on the Senate side of Capitol Hill. The first was on Sept. 26, when one broke out in the Hart Senate Office Building. It happened there again on Sept. 28. And then it really spread: Oct. 3 saw at least three fires in the Hart and Dirksen buildings, a Halloween fire in the Dirksen basement caused an evacuation, and Friday capped off the apparent series with the small restroom fire. (The Capitol Police have changed their assessment of the Oct. 3 fires, which they originally said totaled four.) All except the Halloween fire were in women’s restrooms.

Capitol Police have labeled them all “suspicious” and are actively investigating the causes, said spokeswoman Sgt. Kimberly Schneider. Police also are looking at additional bathrooms where suspicious activity was reported, she said.

The department also has increased foot patrols and amped up security around the Hart and Dirksen buildings. But in all cases, police were able to control the fires and put them out.

“These are all isolated, and the Capitol Police is well prepared to handle any incident at any time,” she said. “What we’d love to do is find this person. It’s lucky this hasn’t caused any injuries to anyone.”

Wednesday’s fire is the only one that caused an evacuation. An Architect of the Capitol staffer pulled a manual trigger for the audible alarm, Schneider said, putting in motion security procedures. However, like the other fires, it was quickly extinguished.

But the evacuation was helpful in at least one way: It helped test out a new system that allows Senate staffers to use their BlackBerrys and laptops to let police know they have evacuated. Hundreds of BlackBerrys have been programmed to link into a database where employees essentially check themselves off a list, said Senate Sergeant-at-Arms Terrance Gainer. It’s a Web-based program specifically made for his office and has been set up for a year, he said.

Employees can still do it the old-fashioned way — that is, meeting the designated office coordinator at a predetermined spot and allowing that coordinator to check them off of a list. But on Wednesday, about 68 percent of employees checked in electronically, Gainer said.

Wednesday’s evacuation went well, Gainer said Thursday, but it also highlighted a few areas where it can be improved.

“There was some minor confusion yesterday about whether it was a self-evacuation and what alarms were sounding, but overall the staff was excellent in responding,” he said. His office will hold an evacuation debriefing to see if any improvements need to be made.