"We went from summer slumber to back-to-school volatility at the flip of a switch," said Art Hogan, chief market strategist at Wunderlich Securities, noting that the market's dilemma is three-fold. "We don't know what the Fed will do. We know they want to raise rates, but the economic calendar is telling them this might not be the right time." He also said concerns about a narrowing race between Hillary Clinton and Donald Trump, as well as a fall in commodity prices, were weighing on market sentiment.

The Dow Jones industrial average briefly fell more than 100 points before closing about 90 points lower, as United Technologies and Goldman Sachs contributed the most losses. At session lows, the Dow fell 142.27 points.

"Investors are taking a look at the next catalyst. I think a potential rate hike is definitely a major catalyst," said Mike Bailey, director of research at FBB Capital Partners, noting he does not expect the U.S. central bank to raise interest rates next week. "The two main things the Fed looks at are jobs and inflation. On jobs, you can check the box; on inflation, we're not so sure."

U.S. stocks closed lower on Friday, but posted weekly gains, as investors digested key inflation data and looked ahead to next week's Federal Reserve meeting.

"Intraday overbought conditions returned with yesterday's rally, supporting a resumption of the pullback today. We are impressed by the ability of the SPX to hold up near 2100-2135 support, but would not rule out a brief dip below this level. Short-term momentum remains weak, and oversold conditions appear likely to expand before a tradable low is established," said Katie Stockton, chief technical strategist at BTIG.



David Schiegoleit, managing director at the Private Client Reserve at U.S. Bank said declining oil prices were also contributing to losses in equities. "The handcuff between oil prices and stocks started to break off in the second quarter," he said, noting that oil and stocks would trade in tandem again if the fall in oil prices "is seen as a reflection of the global economy, ... or as something that could take out the energy sector."



The Nasdaq held about 0.1 percent lower, as Apple slipped about half a percent. Still, the tech giant's stock advanced 11.43 percent for the week.

"I think the real problem here is the CPI number," said Peter Cardillo, chief market economist at First Standard Financial. "Now, I don't think the Fed will go next week, but this is a number they can't ignore for much longer."

"I think the Fed is going to be very vocal in that inflation is creeping up," he said.

Bank stocks were also in focus on Friday as Deutsche Bank's U.S.-listed shares fell 9.42 percent after the U.S. Justice Department suggested the German banking giant pay $14 billion to settle a number of investigations related to mortgage securities. The SPDR S&P Bank ETF (KBE) fell 0.87 percent.

The three major indexes posted weekly gains despite the amount of volatility investors have encountered. The Dow had recorded three tripe-digit moves on a closing basis this week as of Thursday's close, but still managed to gain 0.21 percent for the week. The S&P and the Nasdaq notched weekly gains of 0.53 percent and 2.31 percent, respectively.



Dow Jones 1-month chart

Source: FactSet

U.S. consumer prices increased more than expected in August as rising rents and health care costs offset a drop in gasoline prices, pointing to a steady build-up of inflation that could allow the Federal Reserve to raise interest rates this year.

The Labor Department said on Friday its Consumer Price Index rose 0.2 percent last month after being unchanged in July. In the 12 months through August, the CPI increased 1.1 percent after advancing 0.8 percent in July.

"Janet Yellen certainly applauded the figures, for the lack of any upward pricing pressures has persisted for far too long. Nevertheless, the Chairman and her colleagues will almost assuredly refrain from adding restrictive measures next week given a recent string of poor data," said Jeremy Klein, chief market strategist at FBN Securities.



Market expectations for a rate hike next week rose slightly after the data release, but remained relatively low. According to the CME Group's FedWatch tool, market expectations for a September rate hike rose to 15 percent from 12 percent. Investors have been watching closely the recent data, looking for clues as to whether the Fed will raise rates at its two-day meeting, which starts Tuesday and ends Wednesday.

Other data released Friday included the September consumer sentiment read, which came in below the expected 90.8.

U.S. Treasurys, meanwhile, traded lower following the inflation data release, with the two-year note yield at 0.77 percent and the benchmark 10-year note around 1.69 percent. The U.S. dollar extended gains against a basket of currencies, with the euro near $1.116 and the yen around 102.33.

In oil markets, U.S. crude settled 2 percent lower at $43.03 per barrel as oversupply concerns weighed on the commodity. Baker Hughes reported U.S. drillers increased the number of rigs operating in U.S. fields by 2 in the previous week to a total of 416. At this time last year, drillers had 644 rigs in U.S. fields. The rig count has risen in 11 of the last 12 weeks.