With the game goalless early in the second half, Aaron Ramsey raced on to Olivier Giroud's through ball with Swansea right-back Dwight Tiendalli in pursuit. The Welshman went to ground but referee Howard Webb waved away the Arsenal appeals.
"It was a penalty and the referee saw it," he said. "Do you know why he did not give it? Because he thought the guy had not done it on purpose, and I also think it was accidental. But he (Ramsey) did not trip himself, he was caught clearly by the leg of the Swansea player. The referee hesitated and he did not give it."
Swansea manager Michael Laudrup admitted he had believed at the time it was a penalty, although he had not had a chance to see the incident again. He said: "On the bench my initial reaction was it was a penalty, but I have not seen it again after the game and sometimes you can change your mind when you see things again."
The tie had meandered along with little incident of note during a tepid opening half, but it sprung to life in the second 45 minutes. Michu, who settled last month's league meeting between the sides, had looked to again be Arsenal's downfall as he gave Swansea the lead just 73 seconds after coming on as a second-half substitute.
But two goals in three minutes from Lukas Podolski and Kieran Gibbs put the Gunners on course to advance, only for Danny Graham, who scored a late leveller against Aston Villa in midweek, to rifle home with three minutes left.
Wenger felt his side should have seen the game out after leading late on.
"We should have won this game, the first half was a bit locked and one-paced but the second was much more open," he said. "We created many chances and found ourselves 1-0 down, but we showed character and quality."
Laudrup, who started with top scorer Michu on the bench and rested defender Ashley Williams, felt a draw was a fair result, and again praised the character shown by his side.
"I think the overall feeling is that it was a fair result," he said. "In the second half we made a couple of changes that paid off with Michu's goal, but to be fair to Arsenal they put us under pressure, they kept the ball, took over possession and in those 15 minutes we lost the ball too much."
Source: PA
Source: PA