SANTA CLARA, Calif. -- Just before halftime of Thursday night's game between the San Francisco 49ers and New York Giants, Niners left tackle Trent Williams appeared to throw a right-handed punch to the facemask of Giants defensive tackle A'Shawn Robinson.

The scuffle between Williams and Robinson drew penalty flags for unnecessary roughness on both players, forcing Niners quarterback Brock Purdy to kneel again to end the half. But given the punch, the Giants appeared to wonder why Williams wasn't ejected.

NFL senior vice president of officiating Walt Anderson spoke to a pool reporter after San Francisco's 30-12 win, noting that his crew reviewed the play using available video to see if any further action needed to be taken.

"We ended up looking at the video we had available to us, and we just didn't see anything that rose to the level of flagrant," Anderson said. "Which is the standard that we have to apply to disqualify the player."

Anderson went on to explain that to determine how flagrant a punch is, officials look at whether it was done with a closed fist, an open-hand slap or a stiff-arm to the face. According to Anderson, a closed fist punch "carries a different weight to it."

While Williams appeared to throw a closed-fist punch at Robinson, Anderson said the officials didn't see a replay angle that gave them conclusive evidence that he did.

"We couldn't confirm that 100 percent from the standpoint of was it truly a closed fist with a strike," Anderson said. "We just couldn't determine that."

The play took place with about 13 seconds remaining in the second quarter. San Francisco was kneeling to end the half when Niners left guard Aaron Banks shoved Robinson away. Robinson immediately got in Williams' face, and the two began arguing.

"Just mixing it up," Williams said. "A lot of tempers flaring out there, it's a competitive game and sometimes things boil over a little bit."

Robinson was asked about the incident after the game but declined to offer his side, mostly responding with, "I don't know."

Williams and Robinson have played against each other multiple times over the past three years when Robinson was playing for the Los Angeles Rams. Williams added that he didn't expect to be fined for hitting Robinson.

"I don't think so," Williams said. "It was a love tap. It wasn't that hard."

ESPN's Jordan Raanan contributed to this report.