A pop-up memorial for the victims of Wednesday's attacks in San Bernardino, Calif. Authorities are investigating the shooting as an act of terrorism, FBI Assistant Director David Bowdich said Friday. (Mario Anzuoni/Reuters)

When a gunman attacked a Planned Parenthood facility in Colorado Springs., Colo., late last month, potentially out of opposition to abortion, it was a reminder that right-wing violence had caused more deaths in the United States in recent years than had attacks carried out by Islamist extremists.

The New America Foundation, a Washington think tank, has compiled data on different brands of terrorism in the U.S. since the terrorist attacks of Sept. 11, 2001. The killing of three people in the Planned Parenthood shooting brought the total number of people killed by right-wing extremists, a category that includes white supremacists and anti-government fanatics, to 48. By comparison, 31 people in the United States had been killed by extremists violently promoting a form of government based on Islamic law.

But now, the number of deaths resulting from Islamist-inspired attacks in the U.S. appears to be growing. On Friday, the New America Foundation added Wednesday's massacre in San Bernardino, Calif. — where 14 people were killed and 21 were wounded — to its list of deadly attacks carried out by jihadists, bringing the total to 45 deaths. Federal authorities are investigating the case as an act of terrorism and say one of the killers pledged her allegiance to the leader of the Islamic State on the day of the shooting.

If those suspicions are confirmed, it would make this year the deadliest for Islamist attacks in the United States since 2001, according to New America, with a total 19 deaths. In July, a gunman slaughtered four Marines and a Navy sailor in Chattanooga, Tenn. While no motive has been publicly confirmed, the government was investigating whether he had ties to Islamist terrorist groups.

Here are the deadly attacks by jihadists, as characterized by New America Foundation, since 2001.

And here are the cumulative deaths caused by Islamist attacks and right-wing attacks over the past 13 years.

Since Sept. 11, 2001, 317 individuals in the U.S. have been charged with Islamist terrorism, compared with 182 non-Islamists, according to the data.

The New America database gathers information about people in the U.S. who engage in violent extremist activity and separates them into two groups – those motivated by jihadist ideology, like al-Qaeda and its affiliated groups, and those driven by non-jihadist ideologies, including right-wing and left-wing beliefs.

These figures can change quickly.

Following the shooting death of nine people in an African American church in Charleston, S.C., in June, numerous media outlets cited New America's count, arguing that homegrown extremists had killed far more people than had jihadists. At the time, the data showed that 48 people had been killed by Islamist extremists since 9/11 — nearly twice the number of people who had been killed by self-proclaimed jihadists, 26.

But events since June have substantially changed the tally.

As Megan McArdle has argued for Bloomberg View, the way these events are counted and classified has a huge influence on the data. For example, if the starting date for the figures were set back one day, to include deaths since 9/11, the total would look quite different. Yet, in a country where "terrorist organization" is often synonymous with radical groups such as al-Qaeda or the Islamic State, some people may also find the number of fatalities at the hands of right-wing terrorists surprising.

[When should a shooting really be called 'terrorism'?]

In a description of the database's methodology, New America acknowledges that extremism is a subjective term. It says it has gathered data on any use of violence in the pursuit of political ideology, regardless of whether that ideology is considered to be mainstream in the U.S.

For example, the list of deadly right-wing attacks includes a 2004 Tulsa, Okla. bank robbery, because the perpetrators testified that they were stealing the money to buy weapons and carry out a “mission to revenge Waco.”

It also includes the 2012 shooting of six people in a Sikh temple in Wisconsin by a white supremacist, the murder of three people at Jewish institutions in Kansas City by a man with ties to the Ku Klux Klan and the shooting of nine people at the Emmanuel AME Church in Charleston, S.C. by white supremacist Dylann Roof.

New America's count of deadly jihadist attacks includes the 13 people killed in a shooting at the Ford Hood military base in 2009, a beheading in Oklahoma by an Islamist extremist in September 2014 and the four people killed in the Boston Marathon bombing, among others.

But New America chose to exclude the Beltway sniper of 2002, for example, because "they said something about Osama bin Laden once, but they seemed to be losers who wanted to be the heroes of their own story," Peter Bergen, who directs the International Security Program program at New America, said in an interview.

It's also worth realizing that the numbers are fairly small. One major deadly attack — like this week's — can change the takeaway quickly.

See also: 

-One map shows why America’s gun violence is so much worse than anywhere else

-What the rest of the world wonders about America, according to Google

-How the Islamic State makes its money